What's the difference between int x int y and int x,y

Between:

int x;
int y;

and:

int x, y;

For example, in the next code:

public static boolean sda4(int[] arr) {
	for(int i=0, j=arr.length/2; i<arr.length/2 && j<arr.length; i++, j++)
		if(arr[i]!=arr[j]) return false;
	return true;
}

If I type inside the for loop (int i=0, int j=arr.length/2) it says "syntax error".

There are no differences in bytecode terms.

• With the multiple variable declaration you save some lines and it is easier to read since they all belong to the same data type. The reserved memory space is the same in both cases.

• With the declaration and multiple initialization of variables, although you save lines and the code is more compact, it could be more complex or annoying to read.

Therefore, sometimes the code is more compact does not imply that it is of higher quality.

GL

Let us do a little bit of experimentation, shall we? We take a closer look at three different programs.

Test1.java:

public class Test1 {
    public static void main(String[] args) {
        int x;
        x = 1;
        int y;
        y = 2;
        System.out.println(x);
        System.out.println(y);
    }
}

Test2.java:

public class Test2 {
    public static void main(String[] args) {
        int x = 1, y = 2;
        System.out.println(x);
        System.out.println(y);
    }
}

Test3.java:

public class Test3 {
    public static void main(String[] args) {
        int x;
        int y;
        x = 1;
        y = 2;
        System.out.println(x);
        System.out.println(y);
    }
}

When we now compile those three programms (javac Test1.java Test2.java Test3.java) and take a look at the produced bytecode (javap -C Test[123]), we see the following:

javap -c Test1:

Compiled from "Test1.java"
public class Test1 {
  public Test1();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: iconst_1
       1: istore_1
       2: iconst_2
       3: istore_2
       4: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
       7: iload_1
       8: invokevirtual #13                 // Method java/io/PrintStream.println:(I)V
      11: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
      14: iload_2
      15: invokevirtual #13                 // Method java/io/PrintStream.println:(I)V
      18: return
}

javap -c Test2:

Compiled from "Test2.java"
public class Test2 {
  public Test2();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: iconst_1
       1: istore_1
       2: iconst_2
       3: istore_2
       4: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
       7: iload_1
       8: invokevirtual #13                 // Method java/io/PrintStream.println:(I)V
      11: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
      14: iload_2
      15: invokevirtual #13                 // Method java/io/PrintStream.println:(I)V
      18: return
}

javap -c Test3:

Compiled from "Test3.java"
public class Test3 {
  public Test3();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: iconst_1
       1: istore_1
       2: iconst_2
       3: istore_2
       4: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
       7: iload_1
       8: invokevirtual #13                 // Method java/io/PrintStream.println:(I)V
      11: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
      14: iload_2
      15: invokevirtual #13                 // Method java/io/PrintStream.println:(I)V
      18: return
}

Since the bytecode for all three classes is identical, the question is one of personal preference. In Java, normally each variable is delcared on its own line. Braian Coronel discussed some pros and cons wrt. code readability in his answer

> Fix your code like this,
> Remove 'int' before j=arr.length/2
Because you can't declare more than one variable type inside for loop condition . Because int x,y is the only one thing you can create variables inside loop condition

public static boolean sda4(int[] arr) {
	for(int i=0, j=arr.length/2; i<arr.length/2 && j<arr.length; i++, j++)
		if(arr[i]!=arr[j]) return false;
	return true;
}