Develop Reference

Operator definition in java

int i = 10;
i++; // it ok primitive value can use ++.
Integer integer = 10;
integer++; // how it can use ++
MyClass myClass = new MyClass();
myClass++; // then why myclass can't use ++.

C++ has the ability to overload operators. The Java language considers this to be open to too much abuse (overloaded operators can be obfuscating) so it was never incorporated into Java.
Therefore, you can't write myClass++ as the syntax is not available to specify that operation.
But ++ does work on a selection of non-primitives. The mechanism exploited is called autoboxing. (Essentially the underlying plain-old-data type is extracted from the boxed type, incremented then re-boxed to the original reference type).
Somewhat related to this is the ability to apply += and + to java.lang.String instances. Simply put, this is a special case. Although fully aware of the risk of downvotes I regard this as one of the worst kludges in Java, particularly += which will create a new instance of a string (as strings themselves are immutable), and many Java programmers will be unaware of the effect this has on memory.

It is because of Java's autoboxing feature which is added in Java 1.5
The compiler will convert the statment as follow
Integer integer = 10;
integer.iniValue++;
You can try to add the compiler flag "javac -source 1.4" and it will return an error

From the Link provided in a comment by Konstantin V. Salikhov,
Integer has a defined method to return an int, which then has the ++ operator defined.
MyClass has no ++ operator, hence myClass++; is invalid
The method in question goes like:
Integer myInteger = 10;
myInteger.intValue++;
Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes.
Operator overloading in Java has a description (as to it being not allowed) at Operator overloading in Java

How to use ToString() implicitly in Delphi objects or strings?

Delphi is like an England Queen Guard. It does not like ambiguity and may even kill to protect the hard code. But Java is almost a street corner woman. When I use this is java:
Button button = new Button();
String a = "This is a " + button;
I get This is a button
But if I do that in Delphi:
ShowMessage('This is a ' + Button1);
I get an error, because Delphi has toString() method (now) but it does not implicitly calls it. Because literal strings are not objects in OP. The correct use is:
ShowMessage('This is a ' + Button1.toString());
Is there any way to override this behave so it works like Java?
For reference: How an object will call toString method implicitly?

There's no way to enforce an implicit cast or method call on an object instance.
If this was a record that you controlled then you could implement an Implicit class operator that would perform a cast to string.
The issue discussed link that you refer to is simply an implementation detail of PrintStream.println(). That relies on the ability of String.valueOf() to come up with a string representation for any object which in turn relies on Object.toString(). In fact the discussion there concerning println() is unrelated to the way the + operator in Java works which is the pertinent issue in your question.
Delphi's TObject has a virtual ToString method that could be used to perform the same purpose. So it would be easy enough to use the exact same technique as PrintStream.println() in Delphi code.

If you had a special function that worked like Format, you could get that behavior. I wrote a function like that several years ago, back when the built-in function didn't support Unicode. I've now updated it to support calling ToString on an object when it's passed as the argument for the %s format string. You'd call it like this:
ShowMessage(WideFormat('This is a %s', [Button1]));
I'm sorry the code hasn't really been touched in several years, so it might not work as-is in newer Delphi versions. You'll have to decide whether it's worth the effort to update that code, although it was included in Project Jedi long enough that it did get a few nominal updates to support UnicodeString and 64-bit compilation.

Too bad that it works only on ARM compiler and Delphi.NET
Great article by Nick Hodges in http://edn.embarcadero.com/article/34324
TMyClass = class
class operator Add(a, b: TMyClass): TMyClass; // Addition of two operands of type TMyClass
class operator Subtract(a, b: TMyClass): TMyclass; // Subtraction of type TMyClass
class operator Implicit(a: Integer): TMyClass; // Implicit conversion of an Integer to type TMyClass
class operator Implicit(a: TMyClass): Integer; // Implicit conversion of TMyClass to Integer
class operator Explicit(a: Double): TMyClass; // Explicit conversion of a Double to TMyClass
end;
// Example implementation of Add class operator
TMyClass.Add(a, b: TMyClass): TMyClass;
begin
...
end;
var
x, y: TMyClass
begin
x := 12; // Implicit conversion from an Integer
y := x + x; // Calls TMyClass.Add(a, b: TMyClass): TMyClass
b := b + 100; // Calls TMyClass.Add(b, TMyClass.Implicit(100))
end;

How do I Implement Operators in a Java Class

I am attempting to create a unsigned integer class.
public class UnsignedInteger extends Number implements Comparable<UnsignedInteger>
{
...
}
Is there a way to implement operators such as; +, -, *, /, <<, >>, |, ^, >>>, <<

Java does not support Operator Overloading. The only option you have is define methods like add(), subtract(), multiply(), etc, and write the logic there, and invoke them for particular operation.
You can have a look at BigInteger class to get an idea of how you can define methods to support various operations. And if interested, you can even go through the source code, that you can find in the src folder of your jdk home directory.

There are already 5 answers saying that you cannot overload operators, but I want to point out that you can not use arithmetical operators on objects at all. They only work with primitive types (int, double, etc).
The only reason the following code compiles
Integer a = 1, b = 2;
Integer c = a + b;
is because the Java compiler compiles it as
Integer a = Integer.valueOf(1), b = Integer.valueOf(2);
Integer c = Integer.valueOf(a.intValue() + b.intValue());
If you want this to work for your UnsignedInteger, you have to extend the javac (it is possible, though).

No you cannot override operators in Java.

It's not possible to override operators in Java. What you can do is define methods to represent the operations, like BigDecimal or BigInteger in the standard library do.

There is a javac-plugin (an annotation processor like Lombok) called "Java-OO", which adds operator overloading to Java.
It allows you to add operator overloading to your own classes very easily. In addition to this, many of the built-in classes of the Java API also supports operator overloading when using this plugin.
(For example: Instead of list.get(6) or map.get("hello") you can do list[6] and map["hello"])
All you need to do is to include the .jar on the classpath when compiling with javac.
There are plugins for all major IDEs: Eclipse, Netbeans and IntelliJ IDEA.

No. Java does not support operator overloading.

Java doesnt support operator overloading, they consider it a bad practice, knowing that they overloaded + and += operators for the String class

Java conversion toInteger?

I wonder if there is a way to make an own conversion to integer in java. I mean a solution that is comparable to the implementation of the string conversion (toString). I want my class to be interpreted as integer in equation without calling a special function.
class MyClass(){
private int number;
MyClass(int n){
this.number = n;
}
public int toInteger(){
return number
}
}
usage:
MyClass a = new MyClass(2);
int result = 1+a;
result would be 3.

Java doesn't allow operator overdloading/overriding. You cannot do this.

You're describing operator overloading, and it's not possible in Java. The closest thing would be subclassing Number, but + doesn't work with it. For Strings + works because it has been built in as a special case in the language. There's no way to extend + to work with anything else.
Of course, with your class, int result = 1 + a.toInteger(); works. Just a little extra work.

Java is not C++ you can't overload operators like '+' in Java

This cannot be done in java. If you are curious enough, check this post
Why doesn't Java offer operator overloading? which excplains why it cannot be done and compares between java and c++ approaches

Defining symbols to java class [duplicate]

Please can you tell me if it is possible to overload operators in Java? If it is used anywhere in Java could you please tell me about it.

No, Java doesn't support user-defined operator overloading. The only aspect of Java which comes close to "custom" operator overloading is the handling of + for strings, which either results in compile-time concatenation of constants or execution-time concatenation using StringBuilder/StringBuffer. You can't define your own operators which act in the same way though.
For a Java-like (and JVM-based) language which does support operator overloading, you could look at Kotlin or Groovy. Alternatively, you might find luck with a Java compiler plugin solution.

Operator overloading is used in Java for the concatenation of the String type:
String concat = "one" + "two";
However, you cannot define your own operator overloads.

In addition to all the people pointing out that + is overloaded for Strings, - is also overloaded for both floating point and integer operations, as are * and /.
[edit]
% is also overloaded for floating point, which can be a bit of a surprise for those with a C or C++ background.

Java does not allow operator overloading. The preferred approach is to define a method on your class to perform the action: a.add(b) instead of a + b. You can see a summary of the other bits Java left out from C like languages here: Features Removed from C and C++

As many others have answered: Java doesn't support user-defined operator overloading.
Maybe this is off-topic, but I want to comment on some things I read in some answers.
About readability.
Compare:
c = a + b
c = a.add(b)
Look again!
Which one is more readable?
A programming language that allows the creation of user-defined types, should allow them to act in the same way as the built-in types (or primitive types).
So Java breaks a fundamental principle of Generic Programming:
We should be able to interchange objects of built-in types with objects of user-defined types.
(You may be wondering: "Did he say 'objects of built-in'?". Yes, see here.)
About String concatenation:
Mathematicians use the symbol + for commutative operations on sets.
So we can be sure that a + b = b + a.
String concatenation (in most programming languages) doesn't respect this common mathematical notation.
a := "hello";
b := "world";
c := (a + b = b + a);
or in Java:
String a = "hello";
String b = "world";
boolean c = (a + b).equals(b + a);
Extra:
Notice how in Java equality and identity are confused.
The == (equality) symbol means:
a. Equality for primitive types.
b. Identity-check for user-defined types, therefore, we are forced to use the function equals() for equality.
But... What has this to do with operator overloading?
If the language allows the operator overloading the user could give the proper meaning to the equality operator.

You can't do this yourself since Java doesn't permit operator overloading.
With one exception, however. + and += are overloaded for String objects.

One can try Java Operator Overloading. It has its own limitations, but it worth trying if you really want to use operator overloading.

Just use Xtend along with your Java code. It supports Operator Overloading:
package com.example;
#SuppressWarnings("all")
public class Test {
protected int wrapped;
public Test(final int value) {
this.wrapped = value;
}
public int operator_plus(final Test e2) {
return (this.wrapped + e2.wrapped);
}
}
package com.example
class Test2 {
new() {
val t1 = new Test(3)
val t2 = new Test(5)
val t3 = t1 + t2
}
}
On the official website, there is a list of the methods to implement for each operator !

Or, you can make Java Groovy and just overload these functions to achieve what you want
//plus() => for the + operator
//multiply() => for the * operator
//leftShift() = for the << operator
// ... and so on ...
class Fish {
def leftShift(Fish fish) {
print "You just << (left shifted) some fish "
}
}
def fish = new Fish()
def fish2 = new Fish()
fish << fish2
Who doesnt want to be/use groovy? :D
No you cannot use the compiled groovy JARs in Java the same way. It still is a compiler error for Java.

Unlike C++, Java does not support user defined operator overloading. The overloading is done internally in java.
We can take +(plus) for example:
int a = 2 + 4;
string = "hello" + "world";
Here, plus adds two integer numbers and concatenates two strings. So we can say that Java supports internal operator overloading but not user defined.