Predict the output · Java

String Immutability and Interning

Medium Predict the output Seen at Amazon, Microsoft, Bloomberg

What are the four lines of output?

What does this print?

public class Main {
    public static void main(String[] args) {
        String s = "hello";
        s.toUpperCase();
        s.concat(" world");
        System.out.println(s);

        String t = "hel" + "lo";
        String u = new String("hello");
        System.out.println(s == t);
        System.out.println(s == u);
        System.out.println(s.equals(u));
    }
}

```
HELLO WORLD
true
false
true
```
```
hello
true
false
true
```
```
hello
false
false
true
```
```
hello world
true
true
true
```

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Solution

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The output is:

hello
true
false
true

Strings are immutable: toUpperCase() and concat() return new strings that are thrown away here, so s is still "hello". "hel" + "lo" is a compile-time constant, interned to the same pool object as "hello", so s == t is true. new String(...) always creates a new object, so s == u is false, while equals compares contents.

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What does this print?

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