Master Java Generics: From Basics to Advanced Customizations

1. Introduction to Generics

Because collections like ArrayList store elements as Object, adding mismatched types can cause runtime type errors; generics were introduced to enforce compile‑time type constraints and improve program robustness.

2. Benefits of Using Generics

Improves program robustness and standardization.

Compile‑time checking prevents illegal element insertion.

Reduces the number of casts, enhancing performance.

Allows class fields, method return types, and parameters to be expressed as placeholders.

3. Common Generic Usages

3.1 Generic Interface

Define an interface with type parameters U and R to enforce method signatures.

interface Im<U,R>{
    void hi(R r);
    void hello(R r1,R r2,U u1,U u2);
    default R method(U u){
        return null;
    }
}

Implementations must use the declared types, otherwise compilation fails.

3.2 Generic Collections

Using generics with HashSet and HashMap to store Student objects eliminates casts and enforces type safety.

HashSet<Student> students = new HashSet<>();
students.add(new Student("Lazy",21));
students.add(new Student("Happy",41));
students.add(new Student("Cute",13));
for (Student s : students){
    System.out.println(s);
}

HashMap<String,Student> hm = new HashMap<>();
hm.put("001",new Student("Happy",21));
hm.put("002",new Student("Lazy",32));
hm.put("003",new Student("Cute",43));
Set<Map.Entry<String,Student>> entries = hm.entrySet();
Iterator<Map.Entry<String,Student>> it = entries.iterator();
while(it.hasNext()){
    Map.Entry<String,Student> e = it.next();
    System.out.println(e.getKey()+" - "+e.getValue());
}

The generic K‑V pair in HashMap ensures both key and value types are known, removing the need for manual casting.

4. Details of Generic Usage

4.1 Type Specification in <>

4.2 Inheritance

P<A> ap = new P<A>(new A());
P<A> ap1 = new P<A>(new B()); // B extends A
class A{}
class B extends A{}

4.3 Shorthand Form

P<A> ap = new P(new A());

5. Custom Generics

5.1 Generic Class Used in Method

public static void main(String[] args){
    U<String,Double,Integer> u = new U<>();
    u.hi("hello",1.0);
}
class U<X,Y,Z>{
    public void hi(X x, Y y){}
}

5.2 Generic Method

public static void main(String[] args){
    U<String,Double,Integer> u = new U<>();
    u.m1("xx",22);
}
class U<X,Y,Z>{
    public <X,Y> void m1(X x, Y y){}
}

The compiler automatically boxes primitive arguments to match the declared generic types.

5.3 Pitfalls

Generic arrays cannot be created because the runtime component type is unknown (e.g., A[] a = new A[]; is illegal).

Static methods cannot use class‑level generic type parameters because static context is independent of any instance.