What's the difference between these 2 simply if statements in java-CodePudding

As the question asked, I was working on an assignment about creating the bounding box of shapes. I was trying to make a comparison for the maximum X coordinate but it won't work as expected in the following code.

public class Test{
    public static void main(String []args) {
        Scanner in = new Scanner(System.in);
        double x1;
        double x2;
        double x3;
        double maxX= 0;
        System.out.println("input x1");
        x1 = in.nextDouble();
        System.out.println("input x2");
        x2 = in.nextDouble();
        System.out.println("input x3");
        x3 = in.nextDouble();

            if(x1>=x2) {
                 if(x1>=x3) {
                     maxX = x1;
                 }
                }
             else if(x2>=x1) {
                 if(x2>=x3)
                     maxX = x2;
             }
             else {
                 maxX = x3;
             }

If I put x1 =2,x2=3,x3=5. The returned value would equal zero. Which is the initialized value. But the program would run correctly with the following code.

public class Test{
    public static void main(String []args) {
        Scanner in = new Scanner(System.in);
        double x1;
        double x2;
        double x3;
        double maxX= 0;
        System.out.println("input x1");
        x1 = in.nextDouble();
        System.out.println("input x2");
        x2 = in.nextDouble();
        System.out.println("input x3");
        x3 = in.nextDouble();

           
            
        
        if((x1>=x2)&&(x1>=x3)){
            maxX=x1;
                }
        else if((x2>=x1)&&(x2>=x3)){
            maxX=x2;
            }
        else
            maxX=x3;
        System.out.println(maxX);
    }
}

This time the system would print 5, but I really don't get it. For the other three coordinates, I used the first method and it worked out perfectly. I don't know why this specific maxX method would get a problem.

CodePudding user response：

In the first example your second statement does not do anything for the case in which x2 is greater than one, but lower than x3. You also have the same problem a statement before:

    if(x1>=x2) {
       if(x1>=x3) {
           maxX = x1;
       }
       else {
          //x1 >= x2 && x1 < x3 => x3 is the greatest
       }
    }
    else if(x2>=x1) {
       if(x2>=x3)
          maxX = x2;
       }
       else {
          // x2 >= x1 && x2 < x3 => x3 is the greatest
       }
    else {
       // Can never be reached, as either x1 >= x2 or x2 >= x1
       maxX = x3;
    }

I hope this answer was helpful, I never gave one before.

CodePudding user response：

In the first example you've nested the if-else statements while in the second you're using logic operators. Let's just follow the flow.

In the first example, is x1>=x2? It is not so we drop to the else. Is x2>=x1? It is, so we execute the following statement. Is x2>=x3? It is not so we're done. maxX is not assigned a value. You do not execute the final else because x2>=x1 returned true.

In the second example, is (x1>=x2)&&(x1>=x3) true or false. We know x1>=x2 is false and so the expression is false, which means to drop to the else. Is (x2>=x1)&&(x2>=x3) true or false? x2>=x1 is true but x2>=x3 is false, and so the expression is false. This means we drop to the final else and maxX=x3.

It the difference between evaluating the conditional one after the other, or at the same time.