Detailed explanation of examples of generics in Java-javaTutorial-php.cn

Detailed explanation of examples of generics in Java

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Release： 2017-06-30 10:10:59

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1. Definition

Generic refers to the ability to parameterize types. You can define a class or method with a generic type, and then the compiler replaces it with a concrete type (generic instantiation). The main advantage of using generics is the ability to detect errors at compile time rather than at runtime. It is a feature that appeared after jdk1.5 to enhance security. My understanding is that it is more like a special specification. For example, when the programmer calls it, or when the client introduces it, it cannot be a mixed bag of good and bad. You can do whatever you want? ! The previous definition said that if you enter a String type, if you are disobedient here, there is no need to let you continue the execution, and you will be killed directly.

2. Disadvantages of undetermined generics

1. Warning, no generic parameterization

2. Undefined generics, collections Any type of object can be loaded into it, which is unsafe

3. When fetching the data in the collection, forced transfer is required

import java.util.Iterator;import java.util.Set;import java.util.TreeSet;public class Test {public static void main(String[] args) {
        Set treeSet = new TreeSet();//没有使用泛型，应该这样：Set<Student> treeSet = new TreeSet<Student>();treeSet.add(new Student(11, 80, "李平"));
        treeSet.add(new Student(23, 40, "王芳"));
        treeSet.add(new Student(10, 60, "赵磊"));
        treeSet.add(new Student(12, 40, "王小二"));
        treeSet.add(new Student(10, 60, "马苗"));
        treeSet.add(new Student(18, 60, "马苗"));
        treeSet.add(new Student(25, 70, "姜浩"));

        Iterator it = treeSet.iterator();while (it.hasNext()) {
            Student stu = (Student) it.next();//没有使用泛型：需要强转            System.out.println(stu);
        }

    }
}

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Start at compile time Error (yellow):

After modification:

##Note: Generic types must be reference types! ! !

Note: Generic types must be reference types! ! !

3. Generic formulation

In JDK we often see the following three situations:

1.TreeSet(Collection c)

2.TreeSet(Comparator comparator)

3.TreeSet( SortedSet s)

Among them,

? wildcard refers to any data type

< > refers to is generic. (As can be seen, 3 is the normal definition of generics)

Note below:

The upper limit of generics: // Indicates receiving E Type, or a subtype of E

The lower limit of generics: // Indicates receiving the type E, or the parent type of E

Code example:

//这时这个函数只能接收 Number及其子类static void show(Point<? extends Number> p){ 
        System.out.println(p.getX());
        System.out.println(p.getY());
    }    public static void main(String[] args) {/* 对于上面的声明,下面的操作是可以的
        Point<Integer> p1=new Point<Integer>();
        p1.setX(new Integer(90));
        p1.setY(new Integer(50));
        show(p1);      *///下面的操作将出错Point<String> p1=new Point<String>();
        p1.setX("90ok");
        p1.setY("50ok");
        show(p1); //出错

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  show(Point<?  String>  <String> p1= Point<String>"90ok""50ok"

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(picture from the Internet)

4. Understand the application of generics

We can customize generic classes, generic methods and generic interfaces. When learning, you need to know its principle, and you can happily call it in the JDK in the future~~

1. Custom generic class

class ObjectFactory<T> { // 声明泛型為<T>private T obj;public T getObj() {return this.obj;
    }public void setObj(T obj) {this.obj = obj;
    }/* * 下面的写法不成立 public T getNewObj(){ T t=new T(); //在编译期,无法确定泛型的参数化的类型 return
     * t; }     */}class Test4 {public static void main(String[] args) {// List list=new ArrayList();/* * ObjectFactory f=new ObjectFactory(); f.SetObj("ss");         */ObjectFactory<String> f = new ObjectFactory<String>();
        f.setObj("这里必须是String");// f.SetObj(89); 不可以String obj = f.getObj();
        System.out.println(obj);

        ObjectFactory<Student> stuList = new ObjectFactory<Student>();

        stuList.setObj(new Student(67, 90, "张三"));
        stuList.getObj().speak();
    }
}

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2. Generic methods

public class TestFan {    // 泛型方法,这里不做限制，传什么都可以public <T> void show(T t) {
        System.out.println("这是泛型方法中的" + t);
    }
}class Test5 {public static void main(String[] args) {
        TestFan tfan = new TestFan();
        
        tfan.show("777");
        
        tfan.show(898);
        
        tfan.show(new Student(30, 20, "猫"));
    }
}

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3. Generic interface

5. Generic restrictions

1. You cannot use generic parameters to create instances, that is, you cannot use new E()

2 .The exception class cannot be generic

3. In a static environment, the parameters of the class are not allowed to be generic types (note)

Since all instances of the generic class have the same operation is a class, so the static variables and methods of a generic class are shared by all its instances. Since it is shared, there is no need to redefine the same generic type. If you do not define the same generic type and cannot achieve sharing (or consistency), there is no need to let this situation pass. Therefore, it is illegal to set class parameters to generics in a static environment.

public class Ee<E> {    public static E Example1;  // Illegalpublic static void Example2(E o1) { 
        // Illegal    }static {
        E Example3; // Illegal    }
}

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