Home/Java

WikiQuora Latest Questions

W3spoint99
  • 0
W3spoint99Begginer
Asked: January 1, 2025In:
  • 0

I use x != null to avoid NullPointerException. Is there an alternative? if (x != null) { // ... }

avoidJavanullnullpointerexception
  1. Saralyn
    Saralyn Begginer

    This to me sounds like a reasonably common problem that junior to intermediate developers tend to face at some point: they either don't know or don't trust the contracts they are participating in and defensively overcheck for nulls. Additionally, when writing their own code, they tend to rely on retRead more

    This to me sounds like a reasonably common problem that junior to intermediate developers tend to face at some point: they either don’t know or don’t trust the contracts they are participating in and defensively overcheck for nulls. Additionally, when writing their own code, they tend to rely on returning nulls to indicate something thus requiring the caller to check for nulls.

    To put this another way, there are two instances where null checking comes up:

    1. Where null is a valid response in terms of the contract; and
    2. Where it isn’t a valid response.

    (2) is easy. As of Java 1.7 you can use Objects.requireNonNull(foo). (If you are stuck with a previous version then assertions may be a good alternative.)

    “Proper” usage of this method would be like below. The method returns the object passed into it and throws a NullPointerException if the object is null. This means that the returned value is always non-null. The method is primarily intended for validating parameters.

    public Foo(Bar bar) {
    this.bar = Objects.requireNonNull(bar);
}

    It can also be used like an assertion though since it throws an exception if the object is null. In both uses, a message can be added which will be shown in the exception. Below is using it like an assertion and providing a message.

    Objects.requireNonNull(someobject, "if someobject is null then something is wrong");
someobject.doCalc();

    Generally throwing a specific exception like NullPointerException when a value is null but shouldn’t be is favorable to throwing a more general exception like AssertionError. This is the approach the Java library takes; favoring NullPointerException over IllegalArgumentException when an argument is not allowed to be null.

    (1) is a little harder. If you have no control over the code you’re calling then you’re stuck. If null is a valid response, you have to check for it.

    If it’s code that you do control, however (and this is often the case), then it’s a different story. Avoid using nulls as a response. With methods that return collections, it’s easy: return empty collections (or arrays) instead of nulls pretty much all the time.

    With non-collections it might be harder. Consider this as an example: if you have these interfaces:

    public interface Action {
  void doSomething();
}

public interface Parser {
  Action findAction(String userInput);
}

    where Parser takes raw user input and finds something to do, perhaps if you’re implementing a command line interface for something. Now you might make the contract that it returns null if there’s no appropriate action. That leads the null checking you’re talking about.

    An alternative solution is to never return null and instead use the Null Object pattern:

    public class MyParser implements Parser {
  private static Action DO_NOTHING = new Action() {
    public void doSomething() { /* do nothing */ }
  };

  public Action findAction(String userInput) {
    // ...
    if ( /* we can't find any actions */ ) {
      return DO_NOTHING;
    }
  }
}

    Compare:

    Parser parser = ParserFactory.getParser();
if (parser == null) {
  // now what?
  // this would be an example of where null isn't (or shouldn't be) a valid response
}
Action action = parser.findAction(someInput);
if (action == null) {
  // do nothing
} else {
  action.doSomething();
}

    to

    ParserFactory.getParser().findAction(someInput).doSomething();

    which is a much better design because it leads to more concise code.

    That said, perhaps it is entirely appropriate for the findAction() method to throw an Exception with a meaningful error message — especially in this case where you are relying on user input. It would be much better for the findAction method to throw an Exception than for the calling method to blow up with a simple NullPointerException with no explanation.

    try {
    ParserFactory.getParser().findAction(someInput).doSomething();
} catch(ActionNotFoundException anfe) {
    userConsole.err(anfe.getMessage());
}

    Or if you think the try/catch mechanism is too ugly, rather than Do Nothing your default action should provide feedback to the user.

    public Action findAction(final String userInput) {
    /* Code to return requested Action if found */
    return new Action() {
        public void doSomething() {
            userConsole.err("Action not found: " + userInput);
        }
    }
}
    See less
W3spoint99
  • 0
W3spoint99Begginer
Asked: January 1, 2025In:
  • 0

If I have an object implementing the Map interface in Java and I wish to iterate over every pair contained within it, what is the most efficient way of going through the map? Will the ordering of elements depend on the specific map ...

collectioniterateiterationJavamap
  1. Saralyn
    Saralyn Begginer

    To summarize the other answers and combine them with what I know, I found 10 main ways to do this (see below). Also, I wrote some performance tests (see results below). For example, if we want to find the sum of all of the keys and values of a map, we can write: Using iterator and Map.Entry long i =Read more

    To summarize the other answers and combine them with what I know, I found 10 main ways to do this (see below). Also, I wrote some performance tests (see results below). For example, if we want to find the sum of all of the keys and values of a map, we can write:

    1. Using iterator and Map.Entry 
      long i = 0;
Iterator<Map.Entry<Integer, Integer>> it = map.entrySet().iterator();
while (it.hasNext()) {
    Map.Entry<Integer, Integer> pair = it.next();
    i += pair.getKey() + pair.getValue();
}
    2. Using foreach and Map.Entry 
      long i = 0;
for (Map.Entry<Integer, Integer> pair : map.entrySet()) {
    i += pair.getKey() + pair.getValue();
}
    3. Using forEach from Java 8 
      final long[] i = {0};
map.forEach((k, v) -> i[0] += k + v);
    4. Using keySet and foreach 
      long i = 0;
for (Integer key : map.keySet()) {
    i += key + map.get(key);
}
    5. Using keySet and iterator 
      long i = 0;
Iterator<Integer> itr2 = map.keySet().iterator();
while (itr2.hasNext()) {
    Integer key = itr2.next();
    i += key + map.get(key);
}
    6. Using for and Map.Entry 
      long i = 0;
for (Iterator<Map.Entry<Integer, Integer>> entries = map.entrySet().iterator(); entries.hasNext(); ) {
    Map.Entry<Integer, Integer> entry = entries.next();
    i += entry.getKey() + entry.getValue();
}
    7. Using the Java 8 Stream API 
      final long[] i = {0};
map.entrySet().stream().forEach(e -> i[0] += e.getKey() + e.getValue());
    8. Using the Java 8 Stream API parallel 
      final long[] i = {0};
map.entrySet().stream().parallel().forEach(e -> i[0] += e.getKey() + e.getValue());
    9. Using IterableMap of Apache Collections 
      long i = 0;
MapIterator<Integer, Integer> it = iterableMap.mapIterator();
while (it.hasNext()) {
    i += it.next() + it.getValue();
}
    10. Using MutableMap of Eclipse (CS) collections 
      final long[] i = {0};
mutableMap.forEachKeyValue((key, value) -> {
    i[0] += key + value;
});

    Perfomance tests (mode = AverageTime, system = Windows 8.1 64-bit, Intel i7-4790 3.60 GHz, 16 GB)

    1. For a small map (100 elements), score 0.308 is the best 
      Benchmark                          Mode  Cnt  Score    Error  Units
test3_UsingForEachAndJava8         avgt  10   0.308 ±  0.021  µs/op
test10_UsingEclipseMap             avgt  10   0.309 ±  0.009  µs/op
test1_UsingWhileAndMapEntry        avgt  10   0.380 ±  0.014  µs/op
test6_UsingForAndIterator          avgt  10   0.387 ±  0.016  µs/op
test2_UsingForEachAndMapEntry      avgt  10   0.391 ±  0.023  µs/op
test7_UsingJava8StreamApi          avgt  10   0.510 ±  0.014  µs/op
test9_UsingApacheIterableMap       avgt  10   0.524 ±  0.008  µs/op
test4_UsingKeySetAndForEach        avgt  10   0.816 ±  0.026  µs/op
test5_UsingKeySetAndIterator       avgt  10   0.863 ±  0.025  µs/op
test8_UsingJava8StreamApiParallel  avgt  10   5.552 ±  0.185  µs/op
    2. For a map with 10000 elements, score 37.606 is the best 
      Benchmark                           Mode   Cnt  Score      Error   Units
test10_UsingEclipseMap              avgt   10    37.606 ±   0.790  µs/op
test3_UsingForEachAndJava8          avgt   10    50.368 ±   0.887  µs/op
test6_UsingForAndIterator           avgt   10    50.332 ±   0.507  µs/op
test2_UsingForEachAndMapEntry       avgt   10    51.406 ±   1.032  µs/op
test1_UsingWhileAndMapEntry         avgt   10    52.538 ±   2.431  µs/op
test7_UsingJava8StreamApi           avgt   10    54.464 ±   0.712  µs/op
test4_UsingKeySetAndForEach         avgt   10    79.016 ±  25.345  µs/op
test5_UsingKeySetAndIterator        avgt   10    91.105 ±  10.220  µs/op
test8_UsingJava8StreamApiParallel   avgt   10   112.511 ±   0.365  µs/op
test9_UsingApacheIterableMap        avgt   10   125.714 ±   1.935  µs/op
    3. For a map with 100000 elements, score 1184.767 is the best 
      Benchmark                          Mode   Cnt  Score        Error    Units
test1_UsingWhileAndMapEntry        avgt   10   1184.767 ±   332.968  µs/op
test10_UsingEclipseMap             avgt   10   1191.735 ±   304.273  µs/op
test2_UsingForEachAndMapEntry      avgt   10   1205.815 ±   366.043  µs/op
test6_UsingForAndIterator          avgt   10   1206.873 ±   367.272  µs/op
test8_UsingJava8StreamApiParallel  avgt   10   1485.895 ±   233.143  µs/op
test5_UsingKeySetAndIterator       avgt   10   1540.281 ±   357.497  µs/op
test4_UsingKeySetAndForEach        avgt   10   1593.342 ±   294.417  µs/op
test3_UsingForEachAndJava8         avgt   10   1666.296 ±   126.443  µs/op
test7_UsingJava8StreamApi          avgt   10   1706.676 ±   436.867  µs/op
test9_UsingApacheIterableMap       avgt   10   3289.866 ±  1445.564  µs/op

    Graphs (performance tests depending on map size)

    Enter image description here

    Table (perfomance tests depending on map size)

              100     600      1100     1600     2100
test10    0.333    1.631    2.752    5.937    8.024
test3     0.309    1.971    4.147    8.147   10.473
test6     0.372    2.190    4.470    8.322   10.531
test1     0.405    2.237    4.616    8.645   10.707
test2     0.376    2.267    4.809    8.403   10.910
test7     0.473    2.448    5.668    9.790   12.125
test9     0.565    2.830    5.952   13.220   16.965
test4     0.808    5.012    8.813   13.939   17.407
test5     0.810    5.104    8.533   14.064   17.422
test8     5.173   12.499   17.351   24.671   30.403
    See less
W3spoint99
  • 0
W3spoint99Begginer
Asked: December 31, 2024In:
  • 0

I always thought Java uses pass-by-reference. However, I read a blog post which claims that Java uses pass-by-value. I don’t think I understand the distinction the author is making. What is the explanation?

Javamehodsparameter-passingpass-by-referencepass-by-value
  1. Saralyn
    Saralyn Begginer

    The terms "pass-by-value" and "pass-by-reference" have special, precisely defined meanings in computer science. These meanings differ from the intuition many people have when first hearing the terms. Much of the confusion in this discussion seems to come from this fact. The terms "pass-by-value" andRead more

    The terms “pass-by-value” and “pass-by-reference” have special, precisely defined meanings in computer science. These meanings differ from the intuition many people have when first hearing the terms. Much of the confusion in this discussion seems to come from this fact.

    The terms “pass-by-value” and “pass-by-reference” are talking about variables. Pass-by-value means that the value of a variable is passed to a function/method. Pass-by-reference means that a reference to that variable is passed to the function. The latter gives the function a way to change the contents of the variable.

    By those definitions, Java is always pass-by-value. Unfortunately, when we deal with variables holding objects we are really dealing with object-handles called references which are passed-by-value as well. This terminology and semantics easily confuse many beginners.

    It goes like this:

    public static void main(String[] args) {
    Dog aDog = new Dog("Max");
    Dog oldDog = aDog;

    // we pass the object to foo
    foo(aDog);
    // aDog variable is still pointing to the "Max" dog when foo(...) returns
    aDog.getName().equals("Max"); // true
    aDog.getName().equals("Fifi"); // false
    aDog == oldDog; // true
}

public static void foo(Dog d) {
    d.getName().equals("Max"); // true
    // change d inside of foo() to point to a new Dog instance construct red with name member variable set to "Fifi"
    d = new Dog("Fifi");
    d.getName().equals("Fifi"); // true
}

    In this example, aDog.getName() will still return "Max". The value aDog within main is not changed in the function foo by creating new Dog with name member variable set to "Fifi" because the object reference is passed by value. If the object reference was passed by reference, then the aDog.getName() in main would return "Fifi" after the call to foo.

    Likewise:

    public static void main(String[] args) {
    Dog aDog = new Dog("Max");
    Dog oldDog = aDog;

    foo(aDog);
    // when foo(...) returns, the name of the dog has been changed to "Fifi"
    aDog.getName().equals("Fifi"); // true
    // but it is still the same dog:
    aDog == oldDog; // true
}

public static void foo(Dog d) {
    d.getName().equals("Max"); // true
    // this changes the name of d to be "Fifi"
    d.setName("Fifi");
}

    In this example, Fifi is dog’s name after call to foo(aDog) because the object’s name was set inside of foo(...). Any operations that foo performs on d are such that, for all practical purposes, they are performed on aDog, but it is not possible to change the value of the variable aDog itself.

    See less
W3spoint99
  • 0
W3spoint99Begginer
Asked: December 31, 2024In:
  • 0

If you have a java.io.InputStream object, how should you process that object and produce a String? Suppose I have an InputStream that contains text data, and I want to convert it to a String, so for example I can write that to a log file. What is ...

inputstreamioJavastreamstring
  1. Saralyn
    Saralyn Begginer

    To summarize the other answers, I found 11 main ways to do this (see below). And I wrote some performance tests (see results below): Ways to convert an InputStream to a String: Using IOUtils.toString (Apache Utils) String result = IOUtils.toString(inputStream, StandardCharsets.UTF_8); Using CharStreRead more

    To summarize the other answers, I found 11 main ways to do this (see below). And I wrote some performance tests (see results below):

    Ways to convert an InputStream to a String:

    1. Using IOUtils.toString (Apache Utils) 
       String result = IOUtils.toString(inputStream, StandardCharsets.UTF_8);
    2. Using CharStreams (Guava) 
       String result = CharStreams.toString(new InputStreamReader(
       inputStream, Charsets.UTF_8));
    3. Using Scanner (JDK) 
       Scanner s = new Scanner(inputStream).useDelimiter("\\A");
 String result = s.hasNext() ? s.next() : "";
    4. Using Stream API (Java 8). Warning: This solution converts different line breaks (like \r\n) to \n. 
       String result = new BufferedReader(new InputStreamReader(inputStream))
   .lines().collect(Collectors.joining("\n"));
    5. Using parallel Stream API (Java 8). Warning: This solution converts different line breaks (like \r\n) to \n. 
       String result = new BufferedReader(new InputStreamReader(inputStream))
    .lines().parallel().collect(Collectors.joining("\n"));
    6. Using InputStreamReader and StringBuilder (JDK) 
       int bufferSize = 1024;
 char[] buffer = new char[bufferSize];
 StringBuilder out = new StringBuilder();
 Reader in = new InputStreamReader(stream, StandardCharsets.UTF_8);
 for (int numRead; (numRead = in.read(buffer, 0, buffer.length)) > 0; ) {
     out.append(buffer, 0, numRead);
 }
 return out.toString();
    7. Using StringWriter and IOUtils.copy (Apache Commons) 
       StringWriter writer = new StringWriter();
 IOUtils.copy(inputStream, writer, "UTF-8");
 return writer.toString();
    8. Using ByteArrayOutputStream and inputStream.read (JDK) 
       ByteArrayOutputStream result = new ByteArrayOutputStream();
 byte[] buffer = new byte[1024];
 for (int length; (length = inputStream.read(buffer)) != -1; ) {
     result.write(buffer, 0, length);
 }
 // StandardCharsets.UTF_8.name() > JDK 7
 return result.toString("UTF-8");
    9. Using BufferedReader (JDK). Warning: This solution converts different line breaks (like \n\r) to line.separator system property (for example, in Windows to “\r\n”). 
       String newLine = System.getProperty("line.separator");
 BufferedReader reader = new BufferedReader(
         new InputStreamReader(inputStream));
 StringBuilder result = new StringBuilder();
 for (String line; (line = reader.readLine()) != null; ) {
     if (result.length() > 0) {
         result.append(newLine);
     }
     result.append(line);
 }
 return result.toString();
    10. Using BufferedInputStream and ByteArrayOutputStream (JDK) 
      BufferedInputStream bis = new BufferedInputStream(inputStream);
ByteArrayOutputStream buf = new ByteArrayOutputStream();
for (int result = bis.read(); result != -1; result = bis.read()) {
    buf.write((byte) result);
}
// StandardCharsets.UTF_8.name() > JDK 7
return buf.toString("UTF-8");
    11. Using inputStream.read() and StringBuilder (JDK). Warning: This solution has problems with Unicode, for example with Russian text (works correctly only with non-Unicode text) 
      StringBuilder sb = new StringBuilder();
for (int ch; (ch = inputStream.read()) != -1; ) {
    sb.append((char) ch);
}
return sb.toString();

    Warning:

    1. Solutions 4, 5 and 9 convert different line breaks to one.
    2. Solution 11 can’t work correctly with Unicode text

    Performance tests

    Performance tests for small String (length = 175), url in github (mode = Average Time, system = Linux, score 1,343 is the best):

                  Benchmark                         Mode  Cnt   Score   Error  Units
 8. ByteArrayOutputStream and read (JDK)        avgt   10   1,343 ± 0,028  us/op
 6. InputStreamReader and StringBuilder (JDK)   avgt   10   6,980 ± 0,404  us/op
10. BufferedInputStream, ByteArrayOutputStream  avgt   10   7,437 ± 0,735  us/op
11. InputStream.read() and StringBuilder (JDK)  avgt   10   8,977 ± 0,328  us/op
 7. StringWriter and IOUtils.copy (Apache)      avgt   10  10,613 ± 0,599  us/op
 1. IOUtils.toString (Apache Utils)             avgt   10  10,605 ± 0,527  us/op
 3. Scanner (JDK)                               avgt   10  12,083 ± 0,293  us/op
 2. CharStreams (guava)                         avgt   10  12,999 ± 0,514  us/op
 4. Stream Api (Java 8)                         avgt   10  15,811 ± 0,605  us/op
 9. BufferedReader (JDK)                        avgt   10  16,038 ± 0,711  us/op
 5. parallel Stream Api (Java 8)                avgt   10  21,544 ± 0,583  us/op

    Performance tests for big String (length = 50100), url in github (mode = Average Time, system = Linux, score 200,715 is the best):

                   Benchmark                        Mode  Cnt   Score        Error  Units
 8. ByteArrayOutputStream and read (JDK)        avgt   10   200,715 ±   18,103  us/op
 1. IOUtils.toString (Apache Utils)             avgt   10   300,019 ±    8,751  us/op
 6. InputStreamReader and StringBuilder (JDK)   avgt   10   347,616 ±  130,348  us/op
 7. StringWriter and IOUtils.copy (Apache)      avgt   10   352,791 ±  105,337  us/op
 2. CharStreams (guava)                         avgt   10   420,137 ±   59,877  us/op
 9. BufferedReader (JDK)                        avgt   10   632,028 ±   17,002  us/op
 5. parallel Stream Api (Java 8)                avgt   10   662,999 ±   46,199  us/op
 4. Stream Api (Java 8)                         avgt   10   701,269 ±   82,296  us/op
10. BufferedInputStream, ByteArrayOutputStream  avgt   10   740,837 ±    5,613  us/op
 3. Scanner (JDK)                               avgt   10   751,417 ±   62,026  us/op
11. InputStream.read() and StringBuilder (JDK)  avgt   10  2919,350 ± 1101,942  us/op

    Graphs (performance tests depending on Input Stream length in Windows 7 system)
    enter image description here

    Performance test (Average Time) depending on Input Stream length in Windows 7 system:

     length  182    546     1092    3276    9828    29484   58968

 test8  0.38    0.938   1.868   4.448   13.412  36.459  72.708
 test4  2.362   3.609   5.573   12.769  40.74   81.415  159.864
 test5  3.881   5.075   6.904   14.123  50.258  129.937 166.162
 test9  2.237   3.493   5.422   11.977  45.98   89.336  177.39
 test6  1.261   2.12    4.38    10.698  31.821  86.106  186.636
 test7  1.601   2.391   3.646   8.367   38.196  110.221 211.016
 test1  1.529   2.381   3.527   8.411   40.551  105.16  212.573
 test3  3.035   3.934   8.606   20.858  61.571  118.744 235.428
 test2  3.136   6.238   10.508  33.48   43.532  118.044 239.481
 test10 1.593   4.736   7.527   20.557  59.856  162.907 323.147
 test11 3.913   11.506  23.26   68.644  207.591 600.444 1211.545
    See less