Override toString() in java

Question

Why toString() works? We didn't call it. It works like a constructor.

public class Main {

    public static void main(String[] args) {
    // write your code here
        A a=new A();
        System.out.println(a);
    }
}
class A{
    @Override
    public String toString(){
        return "Hello";
    }
}

what about we will delete println, make breakpoint and run the program in debug mode?

public class Main {

    public static void main(String[] args) {
        A a=new A();
        A a2=a;

    }
}
class A{
    @Override
    public String toString(){
        return "Hello";
    }
}

as we can see, "Hello" sets to a and a2. Why??

Answer 1

When you're debuging code in Java (and many other languages), the IDE evaluates expressions using built-in methods of the same language. The default representation of the object for IDE usually the string representation.

The debugger calls toString method to obtain something to show in variables and since you've overiden it, the debugger shows you Hello instead default description of the object (which is the default implementation of toString in the class Object).

According the java.lang.Object source code, the implementation:

 177:   /**
 178:    * Convert this Object to a human-readable String.
 179:    * There are no limits placed on how long this String
 180:    * should be or what it should contain.  We suggest you
 181:    * make it as intuitive as possible to be able to place
 182:    * it into {@link java.io.PrintStream#println() System.out.println()}
 183:    * and such.
 184:    *
 185:    * <p>It is typical, but not required, to ensure that this method
 186:    * never completes abruptly with a {@link RuntimeException}.
 187:    *
 188:    * <p>This method will be called when performing string
 189:    * concatenation with this object.  If the result is
 190:    * <code>null</code>, string concatenation will instead
 191:    * use <code>"null"</code>.
 192:    *
 193:    * <p>The default implementation returns
 194:    * <code>getClass().getName() + "@" +
 195:    *      Integer.toHexString(hashCode())</code>.
 196:    *
 197:    * @return the String representing this Object, which may be null
 198:    * @throws OutOfMemoryError The default implementation creates a new
 199:    *         String object, therefore it must allocate memory
 200:    * @see #getClass()
 201:    * @see #hashCode()
 202:    * @see Class#getName()
 203:    * @see Integer#toHexString(int)
 204:    */
 205:   public String toString()
 206:   {
 207:     return getClass().getName() + '@' + Integer.toHexString(hashCode());
 208:   }