String equals and == with String concatenation

Question

I am trying to understand the String concatenation with the output of String compare. To be clear, i have the class to compare two strings using == and equals. I am trying to concat the output of == and equals() to a string. The output of equals() concats, but the output of == does not concat. Using boxing feature of java, the boolean concatenated with string will contact. Both equals and == returns boolean to my knowledge. So why is this difference? Can anyone explain on this?

public class StringHandler {

    public void compareStrings() {
        String s1 = new String("jai");
        String s2 = "jai";
        String s3 = "jai";
        System.out.println("Object and literal compare by double equal to :: "
                + s1 == s2);
        System.out.println("Object and literal compare by equals :: "
                + s1.equals(s2));
        System.out
                .println("Literal comparing by double equal to :: " + s2 == s3);
        System.out.println("Literal comparing by equals :: " + s2.equals(s3));
    }

    public static void main(String[] args) {
        StringHandler sHandler = new StringHandler();
        sHandler.compareStrings();
    }
}

output

false
Object and literal compare by equals :: true
false
Literal compareing by equals :: true

Update: Answer

Without parenthesis for s1==s2, the JVM compares the string as "Object and literal compare by double equal to :: jai" == "jai" and the result is false. So the actual content in sysout is not printed. When parenthesis is added the JVM compares the string as "jai" == "jai" and the result is

Object and literal compare by double equal to :: true

Answer 1

When you do

System.out.println("Object and literal compare by double equal to :: "
            + s1 == s2);

you are first concatenating the string "Object and literal compare by double equal to :: " with the string s1, which will give

"Object and literal compare by double equal to :: jai"

then, you are checking if this string is the same object (same reference) than s2:

"Object and literal compare by double equal to :: jai" == "jai"

which will be false (the output will be false).

In other words, it's because operators precedence. One way to "manipulate" operators precedende is to use parentheses. The operations inside parentheses will be parsed first:

System.out.println("Object and literal compare by double equal to :: " + (s1 == s2));

Answer 2

Add parentheses!

  System.out.println("Object and literal compare by double equal to :: " +
                     (s1 == s2));

PLus and other arithmetic ops have higher priority than comparison ops. Using '+' for concat doesn't change that.

Answer 3

problem:

+ s2 == s3

+ sign has higher precedence than == so it will execute(append) it first before executing the == thus giving you only "false"

You need to put parenthesis so it will check for the result of your string compare.

also parenthesis has higher precedence than + sign to it will compare the string first before appending it to the string.

sample:

 System.out.println("Object and literal compare by double equal to :: " + (s1==s2));
    System.out.println("Object and literal compare by equals :: " + (s1.equals(s2)));
    System.out.println("Literal compareing by double equal to :: " + (s2 == s3));
    System.out.println("Literal compareing by equals :: " + (s2.equals(s3)));

operator precedence:

Answer 4

This is an order of operations issue. It's being interpreted as follows (parentheses added) :

System.out.println(("Object and literal compare by double equal to :: " + s1)  == s2);
System.out.println("Object and literal compare by equals :: " + (s1.equals(s2)) );
System.out.println(("Literal compareing by double equal to :: " + s2)  == s3);
System.out.println("Literal compareing by equals :: " + (s2.equals(s3)) );