System.out.println在打印计算结果为布尔值的东西时没有做我期望的事

Question

I'm not sure how to describe my question for a title, so here's my code:

public class NullLogic {

    public static void main(String[] args) {
        NullLogic nl = new NullLogic();

        System.out.println("nl.getNull()  = " + nl.getNull() );

        System.out.println("nl.getNull() == null  = " + (nl.getNull() == null) );
        System.out.println("nl.getNull() == null  = " + nl.getNull() == null );


    }


    private String getNull() {
        return null;
    }

}

Here's the output:

nl.getNull()  = null
nl.getNull() == null  = true
false

Ok---the first line is no problem, exactly what I'd expect. Second line is also good.

Now I've got a problem with the third line, I know the statement that evaluates to a boolean should be in brackets, but I figure I should get a runtime error or something, not a successful execution that suppresses the the string part of the sysout.

What's going on here?

This isn't a serious problem that I need to solve, but I don't know what's going on here and I'd like to know.

Thanks!

Answer 1

System.out.println("nl.getNull() == null  = " + nl.getNull() == null )

is evaluated as

System.out.println(("nl.getNull() == null  = " + nl.getNull()) == null )

ie, first you concatenate two String s (the first is not null - "nl.getNull() == null = " , and the second is null , but even if both were null , concatenating them would result in a not null String ).

The result of this concatenation is the String : "nl.getNull() == null = null" .

Then you compare "nl.getNull() == null = null" to null , which evaluates to false .