Generics

• Generics are parameterized types in Java.
• This allows types like Integer, String or user-defined types to be a parameter to methods, classes or interfaces.

Types

• Generic classes
• Generic methods
• Generic interfaces

Defining

Naming convention

• T – Type
• E – Element
• K – Key
• N – Number
• V – Value

Classes

class Test<T> {
    T obj;
    Test(T obj) { this.obj = obj; } 
    public T useT() { return this.obj; }
}

Multiple types

class Test<T, U>{
    T obj1;  
    U obj2;   

    Test(T obj1, U obj2){
        this.obj1 = obj1;
        this.obj2 = obj2;
    }
}

Methods

class Test {
    static <T> void genericMethod(T element)
    {
//        element...
    }
 

Usage

• Generics work only with reference types

  Test<int> obj = new Test<int>(20); //compile time error
  

• They work with primitive arrays since those are also references.

  ArrayList<int[]> a = new ArrayList<>();
  

• Generic types maintain type safety. Different types behave differently. For example, you can't assign a generic initialized with Integer to a generic initialized with String.

References

• Generics in Java - GFG

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Backlinks

• Predicate (cs)