16

Can I create a class that instantiates with just the = operator, like the String class does? Or is that a feature specific to String class in Java?

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matthew
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    No, you can't. It's something that, as you said, `lies inside Java`. – u32i64 Aug 14 '17 at 07:05
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    And Strings are *not* instantiated with `=`. String constants are defined with `"..."` and it only gets more complicated from there. So, no, you cannot. Use `new`. If you do not like `new`, you can hide it behind a static factory method or something like that. – Florian Schaetz Aug 14 '17 at 07:08
  • https://stackoverflow.com/questions/2068570/how-to-create-a-string-class-replica – soorapadman Aug 14 '17 at 07:09
  • I meant If I Can I tell Java what to do if there is "=" next to my class or "+=". I gave String class just an example of this. Not internals of String class. I get answer by replies with user-defined Operator Overloading. Thanks all. – matthew Aug 14 '17 at 10:16
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    @matthew - Java doesn't actually do anything special for "=" with strings (it does for "+=", but only because it has special behaviour for "+"). – Jules Aug 14 '17 at 17:46
  • If you are interested in this I highly recommend playing around with Groovy. You can run 100% java classes through groovy (it's nearly a perfect superset) but it also allows a lot of new syntax such as operator overloading. I prefer Java for production code, but Groovy is a lot of fun to mess around with. – Bill K Aug 14 '17 at 20:26

3 Answers3

16

No, you can't create a class that's instantiated just with = operator because you can't overload an operator in Java like you can in C++ or C# (see Operator overloading in Java).

Strings are instantiated when you use "something" only if they do not already exist in memory, so you get a reference to the same exact String object each time you write "something".

For example, if you do:

String a = "something";
String b = "something";

Then

a == b; // will be true.

You can take a look at these questions to learn more about how String objects work:

u32i64
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14

Because Java does not support user-defined Operator Overloading a new instance can not be created with = Operator.

Check out Why doesn't Java offer operator overloading? for more information

Manuel Drieschmanns
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7

Code String s = "Hello World!" does not create a new String. It assigns a reference of a String existing in String Pool to s. If the String does not exist in String Pool, then a new String object is created in String Pool, but not with the operator = all alone.

This creates new String objecs:

String s1 = new String("Hello World!"); // new Object
String s2 = new String("Hello World!"); // new Object

System.out.println(s1 == s2); // false

This may or may not create a new String object in String Pool: 

String s1 = "Hello World!";
String s2 = "Hello World!";

System.out.println(s1 == s2); // true

You could get fairly close to the behaviour mentioned above with using getInstance() pattern, consider this:

public class Singleton {
  private Singleton(){}

  private static class SingletonHelper{
    private static final instance INSTANCE = new Singleton();
  }

  public static Singleton getInstance() {
    return SingletonHelper.INSTANCE;
  }
}

Then you could use:

Singleton s = Singleton.getInstance();
kolobezka
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    The first code snippet does not necessarily create a new `String` in the string pool (the strings created by `new String(...)` are not contained in the pool). – Nevay Aug 14 '17 at 15:43
  • @Nevay You are of course right, my fault, corrected my answer. – kolobezka Aug 15 '17 at 06:13