为什么print语句在Java中调用** toString **方法？

Question

Why the following code segment auto calls toString() method?

Code:

public class Demo {
   public Demo() {
    System.out.println(this); // Why does this line automatically calls toString()?
    }

   public static void main(String[] args) {
    Demo dm = new Demo();
   }
}

Answer 1

println is overloaded for various types, the one you are invoking is:

java.io.PrintStream.println(java.lang.Object)

Which looks like this:

public void println(Object x) {
  String s = String.valueOf(x);
  synchronized (this) {
    print(s);
    newLine();
  }
}

And String.valueOf looks like this:

public static String valueOf(Object obj) {
  return (obj == null) ? "null" : obj.toString();
}

So you can see it calls toString() on your object. QED

Answer 2

调用System.out.println（）时，它将尝试在屏幕上打印一些内容（显然）。可以打印的一件事是一个字符串，因此您将方法指定为参数的每个对象都将被“转换”。可以写入输出的内容，因此它调用.toString（）方法

Answer 3

The java code to print the Object

 public void println(Object paramObject)
      {
        String str = String.valueOf(paramObject); // Use valueOf method
        synchronized (this) {
          print(str);
          newLine();
        }
      }

valueOf method of String

public static String valueOf(Object paramObject)
  {
    return paramObject == null ? "null" : paramObject.toString(); // toString() method is called
  }

Answer 4

The designers of Java decided that they wanted to make it nice and simple to print any object at all, using statements like

System.out.println(something);
System.out.print(something);
someOtherPrintWriter.println(something);

without the programmer having to worry too much about what something actually was, so they made lots of versions of those methods. But they couldn't anticipate every possible class that someone might want to print an object of.

But because every class extends Object , either directly or indirectly, all they needed to do was to make any instance of Object printable - which basically meant providing a way to convert any Object to a String .

They did that by including a toString method in the Object class, and making print and println use it. Then, if anyone writes a class and needs it objects to be printed in a particular way, all they need to do is override toString , and then print and println will both do what the programmer expects.