不使用花括号的嵌套IF条件语句的规则

Question

This might be an extremely beginner question given that I have about 2 years of programming experience already, however something always bothered me with nested IF conditional statements and how they work without having curly braces on them.

I've always used the braces as to keep my coding organized. For example like this.

    public static void main(String[] args) 
{
    int x = 9;
    int y = 8;
    int z = 7; 
    if (x > 9)
    {
        if (y > 8)
        {
            System.out.println("x > 9 and y > 8");
        }
    }
    else if (z >= 7)
    {
        System.out.println("x <= 9 and z >= 7");
    }
    else 
    {
        System.out.println("x <= 9 and z < 7");
    }
}

I've always used this pattern as it has always worked for me.

However, why is it that something written in this format doesn't work the same way?

    public static void main(String[] args) 
{
    int x = 9;
    int y = 8;
    int z = 7; 
    if (x > 9)
        if (y > 8)
            System.out.println("x > 9 and y > 8");
    else if (z >= 7)
        System.out.println("x <= 9 and z >= 7");
    else 
        System.out.println("x <= 9 and z < 7");
}

The first code would print out x <= 9 and z >= 7, but the second code would print nothing. (I'm assuming that the else if and else statements are inside the initial if statement).

In other words, what are the rules in how the compiler tests the conditional statements when no curly braces take place like the example above? I've tried looking for information online, but I can't seem to find information and/or I don't know how to specifically call this problem in order to research my doubts.

Answer 1

The compiler does not care about indentation. An else clause is always associated with the last if that doesn't already have an else . This:

if (x > 9)
    if (y > 8)
        System.out.println("x > 9 and y > 8");
else if (z >= 7)
    System.out.println("x <= 9 and z >= 7");
else 
    System.out.println("x <= 9 and z < 7");

is the same as this:

if (x > 9)
{
    if (y > 8)
    {
        System.out.println("x > 9 and y > 8");
    }
    else
    {
        if (z >= 7)
        {
            System.out.println("x <= 9 and z >= 7");
        }
        else
        {
            System.out.println("x <= 9 and z < 7");
        }
    }
}

Also, notice that else if(...) ... really just means else {if(...) {...}} - it's a combination of else and if , not a special feature.

Answer 2

On second if I put braces on it, It will be like this to the compiler. Because else if or else finds immediate if .

if (x > 9) {
        if (y > 8){
            System.out.println("x > 9 and y > 8");
        }else if (z >= 7) {
            System.out.println("x <= 9 and z >= 7");
        }else {
            System.out.println("x <= 9 and z < 7");
        }
}

If you have to replicate you should put at least one braces like:

if (x > 9) {
        if (y > 8)
            System.out.println("x > 9 and y > 8");
        else if (z >= 7)
            System.out.println("x <= 9 and z >= 7");
}else 
    System.out.println("x <= 9 and z < 7");

Now, Last else is off first if statement for sure.

Answer 3

This isn't really a matter of curly braces, it's a matter of how you've formatted your code. Whitespace doesn't matter in Java (or C++.)

So while you've written this:

if (x > 9)
    if (y > 8)
        System.out.println("x > 9 and y > 8");
else if (z >= 7)
    System.out.println("x <= 9 and z >= 7");
else 
    System.out.println("x <= 9 and z < 7");

That doesn't mean it's equivalent to your original code. To illustrate why, here's how this code would be more reasonably formatted:

if (x > 9)
    if (y > 8)
        System.out.println("x > 9 and y > 8");
    else if (z >= 7)
        System.out.println("x <= 9 and z >= 7");
    else 
        System.out.println("x <= 9 and z < 7");

I hope that illustrates what's going on here.

Answer 4

Curly braces are, essentially, to keep the compiler on track. If you only have 1 if, and 1 else, such as:

 if (x < 3)
    // some code here
 else
    // something else here

When you do math problems in real life, order of operations tells you to do what's in the parentheses first, no? The curly brackets are a grouping symbol used to denote what order of operations the compiler is to take.