Java If-Statement 简写形式

Question

I found the following code:

  static void CreatePartys(int n) 
{ 

    int num = 0;
    int[] e = new int[n];
    e[num] = n;

    while (true) 
    { 
         int part = 0; 
        count = count +1;

        while (num >= 0 && e[num] == 1) 
        { 
            part += e[num]; 
            num--; 
        } 

        if (num < 0)  return; 

        e[num]--; 
        part++; 

        while (part > e[num]) 
        { 
            e[num+1] = e[num]; 
            part = part - e[num]; 
            num++; 
        } 

        e[num+1] = part; 
        num++; 
    } 
} 

And I'm stuck at the if-Statement, I've seen this never before, not even inside a Java-Book.

 if (num < 0)  return; 

What does it return? true, false, a number?

Answer 1

After if follows a statement. Following is a valid code:

if (a > 5) b = c + 2;

Here b = c + 2; is a normal statement.

But statement can also defined by braces. You can use braces also in a normal code as follows:

long i = 3;
long j = 4;
long k = 9;

{ i = j + 7; }

{ i = i * k + 123; }

i = 345;

{ k = j * 10; }

{
  i = 234 * j; 
  k = 12 * j;
  System.out.println(i);
}

Such form often makes no sense because it makes the code hard to understand :) But this is valid code which will correctly compile.

What you often see after if is just a statement like above.

Why do we see often braces after if ? After if exactly one statement is executed. If you have code like

if (a > 5)
b = c + 2;
k = i + 3;

then the statement b = c + 2; will be executed if the condition a > 5 holds true. But the statement k = i + 3; will be executed always , independent on a > 5 .

If you want both statements executed only if a > 5 holds true, then you use braces:

if (a > 5) {
  b = c + 2;
  k = i + 3;
}

When you want to execute exactly one statement depending on if , you can omit braces. But your teammates in the project may be unsure if it was your intention to omit the braces or if it was just a mistake. To avoid any such doubts, coding guidelines in many projects require that any statement after if must use braces, no matter how simple the statement is.

Answer 2

Short explanation:

If you just write return; you can stop the execution of the current method.

If statements don't need braces if they have a single statement following them like

if(2<variable)
   System.out.println("inIf")

System.out.println("outsideIf")

So. if (num < 0) return;

Just means: stop execution of method when number is smaller than 0 No other code in the method will be called