Logic of Java Operators && and ||

Question

Hello dear Programmers,

I have a String String input = "30.09.1993"; Then i want to save all numbers in this string in an array(Only numbers!). The "." are at index 2 and 5 so i want to skip these parts of my string in my loop with an if-statement.

I fixed my problem and everything works fine but I'm confused with the logic of the && and || operators.

This is my working code:

String input = "30.09.1993";
int[] eachNumbers = new int[8];

int x = 0;
for(int i = 0; i <= 9; i++){
    if(i != 2 && i != 5){
       eachNumbers[x] = Integer.parseInt(input.substring(i, i+1));
       x++;
    }
}

And this is the code which doesnt work:

String input = "30.09.1993";
int[] eachNumbers = new int[8];

int x = 0;
for(int i = 0; i <= 9; i++){
    if(i != 2 || i != 5){
       eachNumbers[x] = Integer.parseInt(input.substring(i, i+1));
       x++;
    }
}

The only difference between these two code snippets are the operators in the if-clause.

I thought that the results for these operators are:

&& operator:

false + false = false
true  + false = false
false + true  = false
true  + true  = true

|| operator:

false + false = false
true  + false = true
false + true  = true
true  + true  = true

So in my opinion the second code snippet should work and the first should throw a NumberFormatException. But thats not the case.

I'm sure there are some better solutions for what im doing but my question is only about the logic in my if-statement. Can someone explain me the logic behind this? I'm totally confused and thankful for every helping answer.

Greetings Lukas Warsitz

Answer 1

In the second snippet, the if-clause will always be true, because i will always be not 2 or not 5 (because it cannot be 2 and 5 at the same time)

For what you want to do your first snippet is fine, it does exactly what you want: If your are not at the third element and neither at the 6th element, you want to parse, else you are at one of the points in the date.

Answer 2

Key point is here : In && if first case is false, second case won't check.

true && false // Evaluates false because the second is false 
false && true // Evaluates false because the first is false 
true && true // Evaluates true because both are true 
false && false // Evaluates false because the first is false

and

In || if first case is true , second case won't check.

true || false // Evaluates true because the first is true 
false || true // Evaluates true because the second is true 
true || true // Evaluates true because the first is true
false || false // Evaluates false because both are false