Generic mechanism and enhanced for loop

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Catalog

First understand the generic mechanism

Custom generics

enhance for loop （foreach）

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First understand the generic mechanism

️JDK5.0 New features introduced later ： Generic
️ Generic This grammatical mechanism , Only works during the compilation phase of the program , Just for compiler reference ( Runtime generics are useless )
️ Used Generic benefits What is it? ？
    First of all ： The element types stored in the collection are unified .
    second ： The element type taken from the collection is the type specified by the generic , There is no need to do a lot of “ Move down ”！

️ The disadvantages of generics What is it? ？
    Leads to a lack of diversity in the elements stored in the collection ！

️ example 1： The usage of generics

For generics, the scope is reduced , For a set, we can add any type of element ; When we call the iterator next() When the method is used , The type taken out is actually Object type ; We need to transform downward to call the methods in the class ; But if we specify a generic scope , There is no need to transform downward , Accessing the types of other ranges will result in an error ！ Let's feel it through an example ！

package com.bjpowernode.javase.collection;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;


public class GenericTest01 {
    public static void main(String[] args) {
        //1. Don't use generics 
        List mylist = new ArrayList();

        //1.1. Create objects 
        Cat c = new Cat();
        Bird b = new Bird();

        //1.2. Add objects to the collection 
        mylist.add(c);
        mylist.add(b);

        //1.3. Acquisition iterator , Traversal collection call move Method 
        Iterator it = mylist.iterator();
        while(it.hasNext()){
            //  The default object is Object type ;Object There's no move() Method 
            Object obj = it.next();
            //  Move down 
            if(obj instanceof Animal){
                Animal animal = (Animal)obj;
                animal.move();
            }
        }


        //2. Use generics 
        //  Use generics List<Animal> after , Express List Only storage is allowed in the collection Animal Data of type .
        //  Use generics to specify the data types stored in the collection .
        List<Animal> mylist = new ArrayList<Animal>();

        //2.1. Create objects 
        Cat c = new Cat();
        Bird b = new Bird();

        //2.2. Add elements 
        mylist.add(c);
        mylist.add(b);

        //2.3. Acquisition iterator , Traversal call move Method 
        //  This represents iterator iteration Animal type .
        Iterator<Animal> it = mylist.iterator();
        while(it.hasNext()){
            //  Use generics <Animal>, The extracted element is Animal type 
           Animal animal = it.next();
           //  There's no need to move down , Call directly 
            animal.move();
          //   However, calling methods specific to subclasses still requires a downward transformation , It just facilitates a link 
          if(animal instanceof Cat){
              Cat cat = (Cat)animal;
              cat.catchMouse();
          }
          if(animal instanceof Bird){
              Bird bird = (Bird)animal;
              bird.fly();
          }

        }
    }

}

class Animal {
    //  Parent class's own method 
    public void move(){
        System.out.println(" Animals are moving ！");
    }
}

class Cat extends Animal {
    //  Unique method 
    public void catchMouse(){
        System.out.println(" Cat catches mouse ！");
    }
}

class Bird extends Animal {
    //  Unique method 
    public void fly(){
        System.out.println(" The birds are flying ！");
    }
}

️ example 2： Automatic type inference

JDK8 And then introduced ： Automatic type inference mechanism .（ Also known as diamond expression ）

package com.bjpowernode.javase.collection;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class GenericTest02 {
    public static void main(String[] args) {
        // ArrayList< The type here automatically infers >(), Premise is JDK8 After that .
        List<Animal> myList = new ArrayList<>(); // Automatic type inference 

        // Add objects to the collection 
        myList.add(new Animal());
        myList.add(new Cat());
        myList.add(new Bird());

        // Traverse 
        Iterator<Animal> iterator = myList.iterator();
        while(iterator.hasNext()){
            Animal an = iterator.next();
            an.move();
        }

        //  In such as ： Only exist String A collection of types 
        List<String> str = new ArrayList<>(); // Automatic type inference 
        str.add("http://www.126.com");
        str.add("http://www.baidu.com");
        str.add("http://www.bjpowernode.com");
        //str.add(new String(123)); // error , Type mismatch 

        System.out.println(str.size()); //3

        //  Traverse 
        Iterator<String> it2 = str.iterator();
        while(it2.hasNext()){
            //  If you don't use generics , We also need to judge whether it is a String
            Object obj = it2.next();
            if(obj instanceof String){
                String ss = (String)obj;
                ss.substring(7);
            }
             
            //  Use generic expressions , Directly through the iterator String Data of type 
            String s = it2.next();
            //  Call directly String Class substring Method intercepts the string .
            String newString = s.substring(7);
            System.out.println(newString);
        }


    }
}

Custom generics

️ With the above understanding of generics , Can we make our own defined classes have the feature of generics ？ Let's customize generics ！
️ When you customize generics ,<> In angle brackets is an identifier , Write casually .
    java What often appears in the source code is ：<E> and <T>
    E yes Element Word initials .
    T yes Type Word initials .

package com.bjpowernode.javase.collection;
// Custom generics 
public class GenericTest03<E> { //E Just the identifier , Write casually , General writing E perhaps T

    // Provide a method , Pay attention to the internal parameters 
    public void doSome(E e){
        System.out.println(e);
    }
    // Program entrance 
    public static void main(String[] args) {
        //1、 Create objects , If this method can only be called String type 
        GenericTest03<String> gt = new GenericTest03<>(); // Automatic type inference 
        //gt.doSome(10); // Type mismatch 
        gt.doSome("abc"); // This method can only pass parameters with String type 

        //  Create objects , If this method can only be called Integer type 
        GenericTest03<Integer> gt2 = new GenericTest03<>();
        //gt2.doSome("abc"); // Type mismatch 
        gt2.doSome(100); // Automatic boxing 

        //2、 Define a method , The returned type should be consistent with the generic type of the class 
        MyIterator<String> mt = new MyIterator<>();
        String str = mt.get(); // The return type can only be String

        MyIterator<Animal> mt2 = new MyIterator<>();
        Animal an = mt2.get(); // The return type can only be Animal

        //3、 We don't need to define generics , The parameter is Object type 
        GenericTest03 gt3 = new GenericTest03();
        gt3.doSome(new Object()); // The parameter is Object type 

    }
}
class MyIterator<T>{
    public T get(){
        return null;
    }
}

enhance for loop （foreach）

️JDK5.0 Then a new feature was introduced ： be called enhance for loop , Or called foreach

️ We have learned for loop , So what is enhancement for Circulation? ？ Let's first look at grammar ：

for( Element type   Variable name  :  Array or collection ){
      System.out.println( Variable name );
}

 ️ enhance for Cyclic shortcoming ： No subscript ！

package com.bjpowernode.javase.collection;

public class ForEachTest01 {
    public static void main(String[] args) {
        // 1. Define an array 
        int[] arr = {22,3,9,5,6,8};
        // 2. Traversal print （ Ordinary for loop ）
        for (int i = 0; i < arr.length; i++) {
            System.out.println(arr[i]);
        }
        // 3. enhance for loop 
        for(int i:arr){
            // foreach There is a drawback ： No subscript . In a loop that requires subscripts , Enhancement is not recommended for loop .
            // i It's the elements in the array （ Every element in the array ）
            System.out.println(i);
        }
    }

}

️ example ： Use together foreach

For sets with subscripts , Now we have learned three printing methods ： iterator 、 Use subscripts 、foreach

package com.bjpowernode.javase.collection;

import java.util.*;

// Use together foreach
public class ForEachTest02 {
    public static void main(String[] args) {
        //  establish List aggregate 
        List<String> strList = new ArrayList<>();

        //  Add elements 
        strList.add("hello");
        strList.add("world");
        strList.add("kitty");

        //1.  Traverse , Using Iterators 
        Iterator<String> it = strList.iterator();
        while(it.hasNext()){
            System.out.print(it.next()+" ");
        }
        System.out.println();

        //2.  Use subscript mode 
        for (int i = 0; i < strList.size(); i++) {
            System.out.print(strList.get(i)+" ");
        }
        System.out.println();

        //3.  Use foreach
        //  Because generics use String type , So it is ：String s
        for (String s:strList) {
            System.out.print(s+" ");
        }
    }

}

Conclusion

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