What is generics- Introduction to generics

Hello everyone , I meet you again , I'm your friend, Quan Jun .

Catalog

• 1、 What is generics ?
• 2、 How generics are compiled
  • Compilation mechanism of generics ： Erasure mechanism

1、 What is generics ?

Generics are actually passing types as parameters , Generics allow programmers to write code using types that are specified later , When instantiating the class, pass the desired type as a parameter , To indicate these types .

Why introduce generics ？

for example ： Implement a sequence table by yourself

public class MyArrayList { 
   
    public int[] elem;
    public int usedSize;

    public MyArrayList() { 
   
        this.elem = new int[10];
    }
    // add Method 
    public void add(int val) { 
   
        //  Capacity expansion will not be considered for the time being , This is just to talk about generics 
        elem[usedSize++] = val;
    }
    // get Method 
    public int get(int pos) { 
   
        return elem[pos];
    }
}

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You can see here , When using the custom order table above , We Only add int type The elements of , We know java Sequence table in set , You can add Any kind of The data of , How to achieve it ？ Here we try to make int Type changed to Object type , This ensures that any type can be passed in .

public class MyArrayList { 
   
    public Object[] elem;
    public int usedSize;

    public MyArrayList() { 
   
        this.elem = new Object[10];
    }
    // add Method 
    public void add(Object val) { 
   
        //  Capacity expansion will not be considered for the time being , This is just to talk about generics 
        elem[usedSize++] = val;
    }
    // get Method 
    public Object get(int pos) { 
   
        return elem[pos];
    }
}

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main Method to add data to an object , You can add any type of data . however , When data needs to be retrieved , Because the return is Object type , Strong conversion is required to receive... With the corresponding type , Very trouble .

public static void main(String[] args) { 
   
        MyArrayList myArrayList = new MyArrayList();
        myArrayList.add(1);
        myArrayList.add("hello");
        int array1 = (int)myArrayList.get(0);
        String array2 = (String)myArrayList.get(1);
    }

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So here's the question , Do you have to turn around every time to receive it , Can we not make a strong turn ？ At this time, we think of creating an instance object , You can pass the desired type as a parameter , Let this object store all the data of the incoming type , So when you take it out , You can make it clear that all data in the object is of this type , There's no need for forced rotation . This introduces generics .

public class MyArrayList<E> { 
   
	//  When writing a program , Do not specify a specific type , While using <E> there E For the time being 
    //  The specific type is passed in when instantiating the object 
    public E[] elem;
    public int usedSize;

    public MyArrayList() { 
   
    	//  The writing here is not particularly accurate , The reflection mechanism should be used , Let's start here 
        this.elem = (E[])new Object[10];
    }
    // add Method 
    public void add(E val) { 
   
        //  Capacity expansion will not be considered for the time being , This is just to talk about generics 
        elem[usedSize++] = val;
    }
    // get Method 
    public E get(int pos) { 
   
        return elem[pos];
    }
}

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public static void main(String[] args) { 
   

        MyArrayList<Integer> myArrayList1 = new MyArrayList<>();
        myArrayList1.add(1);
        myArrayList1.add(3);
        // myArrayList1.add("world");//  There will be an error , Because the incoming data is not of the specified type , So generics also have the function of automatically checking types 
        int I1 = myArrayList1.get(0);
        
        MyArrayList<String> myArrayList2 = new MyArrayList<>();
        myArrayList2.add("hello");
        myArrayList2.add("world");
        String s1 = myArrayList2.get(0);
    }

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This ensures that any type of data can be passed in , At the same time, you don't need to turn when you take it out ！ The significance of generics ： 1、 Automatically check the type 2、 Automatically cast types

So here MyArrayList What is the type of the corresponding object ？ yes MyArrayList< Integer > Like that ？

You can see here , The types of objects created by the instance are MyArrayList, and <> The content in does not participate in the composition of generic types , that <> Where are the types inside ？ It's time to understand how generics work .

2、 How generics are compiled

An important difference between arrays and generics is how they force type checking . say concretely , Arrays store and check type information at run time . however , Generics check for type errors at compile time , And there is no type information at run time .

Compilation mechanism of generics ： Erasure mechanism

At compile time , take MyArrayList Medium E It's rubbed Object type .

stay main The method has been rubbed into MyArrayList type .

Reference resources : stay java Create a generic array

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