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string interning

In java, as well known, strings are stored in a cache. This technique of optimization is called string interning. To use memory effectively, all the same strings that encounters in a source code will refer to the same object during execution.

Give a look at the code below:

String s1 = "hi";
String s2 = new String("h") + "i";
System.out.println(s1.equals(s2));

false

The first string s1 is known to the compiler, the second string s2 is created in runtime hence the compiler does not apply the optimization and there are two different objects with type string.

By this way, all strings “hi” that are known to the compiler (created with new operator or set explicitly) will refer to the same object. We can change the first object s1 and will see that other strings with the same value will be affected.

Field field = s1
                .getClass()
                .getDeclaredField("value");
field.setAccessible(true);

Field modifiers = Field
                    .class
                    .getDeclaredField("modifiers");
modifiers.setAccessible(true);
modifiers.setInt(field, field.getModifiers() & ~Modifier.FINAL);

field.set(s1, "bye".toCharArray());

After it, all string objects with value “hi” refers to the string object with value “bye” 😃.

System.out.println(s1.equals("bye"));

true