swapping of object in java
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I have studied that java is pass by reference but when I execute following code the strings are not swapped in main method why?

    static void swap(String s1,String s2){
		String temp = s1;
		s1=s2;
		s2=temp;
	}
	
    public static void main(String[] args) {
		String s1="Hello",s2="world";
		swap(s1,s2);
		System.out.println(s1+s2);
    }
	