Defining generics, when insightful, is also simple. But it's good to have a more proficient level of proficiency in reading the source code.

What you need to pay attention to when using generics:

• Instance class sharing

When the program runs, generic classes are shared by all instances of such classes. Because in fact, generics are erased in compiled bytecode classes. For example, ArrayList and ArrayList types. They share a Class instance of the same ArrayList class at runtime.

ArrayList<String> str = new ArrayList<String>();
ArrayList<Integer> integer = new ArrayList<Integer>();

//The answer is true.
System.out.println("str.getClass and integer.getClass Is it equal?"+(str.getClass() == integer.getClass()));

• Illegal method overload

The compiler does not allow two methods with the same name to be defined in a class, with the same type of class as the parameter, otherwise the compiler will report errors.

• instanceof error operation

The instanceof operation cannot be used for exact generics, otherwise compilation errors will occur.

• Mandatory Type Conversion of Generics

Generic types cannot be used for mandatory type conversion, otherwise there are security risks. A ClassCastException exception occurs at runtime with the following inexpensive warnings

public static void main(String[] args) {
        Collection collection = new ArrayList<String>();
        collection.add("hello");

        ArrayList<Integer> integers = (ArrayList<Integer>) collection;
        integers.add(666);
        for (Integer integer : integers) {
            System.out.println(integer);
        }
    }

Operation results:

Generics of Java Collections

List<String> list = new ArrayList<String>();

Generic classes, generic interfaces, and generic arrays

Use the'extends'keyword to qualify type parameters; use the wildcard character'?' to refer to any data type;

public class JavaTest<K extends Integer, V, T>{
    private Map map = new HashMap<K, V>();
    private T[] t;


    public static void main(String[] args) {
        JavaTest java1 = new JavaTest<Integer, String, String>();   
        java1.putMap(1, "hello");
        java1.putMap(2, "world");
        System.out.println(java1.getMap().get(1));

        Integer[] ints = new Integer[] {666, 518};
        JavaTest java2 = new JavaTest<Integer, String, Integer>();
        java2.setArray(ints);
        for (Integer integer : ints) {
            System.out.println(integer);
        }

    }


    public void putMap(K k, V v) {
        map.put(k, v);
    }

    public Map<K, V> getMap() {
        return this.map;
    }


    public void setArray(T[] t) {
        this.t = t;
    }

    public T[] getArray() {
        return this.t;
    }

}

generic method

public class Dollar{

    //generic method printArray
    public static <E> void printArray(E[] args) {
        for (int i = 0; i < args.length; i++) {
            System.out.println("Print Array Content->>>"+args[i]);
        }
    }

    //generic method printMax
    public static <T extends Comparable<T>> T printMax(T x, T y) {

        return x.compareTo(y) > 0 ? x : y;
    }

    public static void main(String[] args) {
        Integer[] ints = {518, 666, 668};
        printArray(ints);

        System.out.println(printMax(ints[1], ints[0]));
    }
}