Is There a Difference Between X++ and ++X in Java

Is there a difference between x++ and ++x in java?

++x is called preincrement while x++ is called postincrement.

int x = 5, y = 5;

System.out.println(++x); // outputs 6
System.out.println(x); // outputs 6

System.out.println(y++); // outputs 5
System.out.println(y); // outputs 6

Diffrence between x++ and ++x?

++x: increment x; the value of the overall expression is the value after the increment

x++: increment x; the value of the overall expression is the value before the increment

Consider these two sections:

int x = 0;
System.out.println(x++); // Prints 0
// x is now 1

int y = 0;
System.out.println(++y); // Prints 1
// y is now 1

I personally try to avoid using them as expressions within a larger statement - I prefer standalone code, like this:

int x = 0;
System.out.println(x); // Prints 0
x++;
// x is now 1


int y = 0;
y++;
System.out.println(y); // Prints 1
// y is now 1

Here I believe everyone would be able to work out what's printed and the final values of x and y without scratching their heads too much.

There are definitely times when it's useful to have pre/post-increment available within an expression, but think of readability first.

difference between X x = new X(); and x = new X();

Assuming that X is a class with a no-arg constructor (for example a default constructor), the following works:

    // Declare and initialize x
    X x = new X();
    // Assign new value to x
    x = new X();

The first code line declares a variable x and assigns a reference to the new instance of X to it (a new X object). The second line assigns a new X instance to the already declared variable x (thus discarding the reference to the first object). We declare a variable by putting the type name (or the word var) before them. So since in the first line, we have X x, this is a declaration.

In Java, variables need to be declared before they are first used. So the first code line would not work without the type name X at the front. Java would complain that the variable x had not been declared.

On the other hand, we are only allowed to declare each variable once. So putting type name X before the second line would be an error too. My Eclipse says Duplicate local variable x because it “thinks” that I am trying to declare a second variable also named x, which is not allowed (for good reasons).

Difference in implementation of x = x + 1 and x++

My professor recently said that although x = x + 1 and x++ will obviously give the same result

I guess your professor perhaps meant - the value of x after x = x + 1 and x++ will be same. Just to re-phrase, as it seems to be creating confusion in interpreting the question.

Well, although the value of x will be same, they are different operators, and use different JVM instructions in bytecode. x + 1 uses iadd instruction, whereas x++ uses iinc instruction. Although this is compiler dependent. A compiler is free to use a different set of instructions for a particular operation. I've checked this against javac compiler.

For eclipse compiler, from one of the comment below from @Holger:

I just tested it with my eclipse and it produced iinc for both expressions. So I found one compiler producing the same instructions

You can check the byte code using javap command. Let's consider the following class:

class Demo {
    public static void main(String[] args) {
        int x = 5;

        x = x + 1;
        System.out.println(x);

        x++;
        System.out.println(x);
    }
} 

Compile the above source file, and run the following command:

javap -c Demo

The code will be compiled to the following bytecode (just showing the main method):

 public static void main(java.lang.String[]);
   Code:
      0: iconst_5
      1: istore_1
      2: iload_1
      3: iconst_1
      4: iadd
      5: istore_1
      6: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      9: iload_1
     10: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
     13: iinc          1, 1
     16: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
     19: iload_1
     20: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
     23: return

Any difference between `var x` and `int x`?

There is no run-time difference. var is compile-time syntactical sugar. If you replace var with the inferred type (int) you get identical results.

Is there any difference between (null != x) and (x != null)

You're going to want to use the second one.

Both do the same exact thing...compare A to B, or compare B to A - both mean the same thing.

It just makes more sense to use x != null because that is more like how we would say this.

You could ask me "Is x not null?" That seems more natural than "Is null not x?" It makes more sense to us.

What is the difference between int[] x; and int x[];?

There is no difference in semantics. Both syntaxes mean the same. Some extract from the JLS §10.2:

The [] may appear as part of the type at the beginning of the
declaration, or as part of the declarator for a particular variable,
or both, as in this example:

byte[] rowvector, colvector, matrix[];

This declaration is equivalent to:

byte rowvector[], colvector[], matrix[][];

However, as Voo states in the below comments, there can be some tricky confusion about these declarations when declaring several arrays in a single statement.

What is the difference between int x = (int) x and x = (int) x in java if I assign double x = 2.3?

You're re-declaring x, so in the line

int x = (int)x

The x on the right is not the same x you previously declared. So its value defaults to 0 so (int)x is 0. You normally wouldn't be able to have two x variables. That's a thing jshell lets you do, but in normal Java you couldn't do.

See this:

jshell> double x = 2.3
x ==> 2.3

jshell> int y = (int)x
y ==> 2

Is there any difference between Object[] x and Object x[]?

Both are legal and both work. But placing [] before the array's name is recommended.

From Javadocs:

You can also place the square brackets after the array's name:

float anArrayOfFloats[]; // this form is discouraged

However, convention discourages this form; the brackets identify the array type and should appear with the type designation.

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