Question

Please help me understand this code. I am new to java.

// C.java
class C { 
  public static void main(String arg[]) { 
    System.out.println("A"+new C()); 
  } 
  public String toString() { 
    System.out.print("B"); 
    return "C"; 
  } 
}
// output:
// BAC  

Answer 1

You need understand 2 concepts here: Java left-to-right evaluation rule and side effect.

"A"+new C()

following the same rule. It gets “A” first, which is a String literal, put it somewhere. Then it evaluate

new C() 

it construct a C Object first, then invoke toString() method of C Object, and gets the value of C object, which is “C“, then concatenates “A” and “C” together, and println “AC“.

Inside the toString() method of C Object, there is a System.out.print("B");
which is invoked when Java evaluate the above expression. It is printed out before the evaluation completed.
That is why “B” is printed first