为什么没有调用A类中的toString？

Question

public class Test {
    public static void main(String[] args) {
        Object a1 = new A();
        Object a2 = new Object();
        System.out.println(a1);
        System.out.println(a2);
    }
}

class A {
    int x;

    public String toString() {
        return "A's x is " + x;
    }
}

Output

A's x is 0

java.lang.Object@1edf1c96

When a1 is printed, the toString() method inside class A is called without being explicitly called. Can you please explain to me how that happens.

Answer 1

The toString() is implicitly called by println(Object) . Obviously, because it needs to display a String , and toString() is guaranteed to return some form of String representation for all objects.

Answer 2

As you can read in the documentation of PrintStream.println(Object) :

public void println(Object x)

Print an Object and then terminate the line. This method behaves as though it invokes print(Object) and then println() .

Parameters:
x - The Object to be printed.

Now if we take a look at PrintStream.print(Object) , we see:

public void print(Object obj)

Print an object. The string produced by the String.valueOf(Object) method is translated into bytes according to the platform's default character encoding, and these bytes are written in exactly the manner of the write(int) method.

Parameters:
obj - The Object to be printed

Now as you can see, internally the String.valueOf(object) is called, which is defined as:

public static String valueOf(Object obj)

Returns the string representation of the Object argument.

Parameters:
obj - an Object.
Returns:
if the argument is null , then a string equal to "null" ; otherwise, the value of obj.toString() is returned.

So that means that if you write something like:

System.out.println(foo);

You actually have written something equivalent to:

System.out.print(foo);
System.out.println();

which expand further into:

System.out.print(String.valueOf(foo));
System.out.println();

which expands into:

if(foo == null) {
    System.out.print("null");
} else {
    System.out.print();
}
System.out.println();

Or to make it more clear. In the code of PrintStream , you will see something like:

public class PrintStream {

    // ...

    public void println(Object x) {
        this.print(x);
        this.println();
    }

    public void print(Object obj) {
        String result = String.valueOf(obj);
        // ... do something with result (print it to the stream)
    }

}

and in String you will find something like:

public class String {

    // ...

    public static String valueOf (Object obj) {
        if(obj == null) {
            return "null";
        } else {
            
        }
    }

}

Answer 3

The toString() method is defined in every Object . Its default behavior is to return a string representation of that object like "java.lang.Object@1edf1c96" .

In your A class you just replaced this behavior with a new one, by redefining the toString() method ("redefining" means defining that method again in a subclass using the same signature).

This is called Override .

When you print an object, the println() method will blindly call the toString() method of the object, without knowing (or caring) if it was redefined or not.

Answer 4

If you want to represent any object as a string, toString() method comes into existence.

The toString() method returns the string representation of the object.

If you print any object, java compiler internally invokes the toString() method on the object. So overriding the toString() method, returns the desired output, it can be the state of an object etc. depends on your implementation.

By overriding the toString() method of the Object class, we can return values of the object, so we don't need to write much code.