Question

   // Here i have list

//如何在对象数据更改后阻止修改列表我希望有一个未经修改的原始列表而不更新它的数据。我还使用了Arraylist和Collection.unmofifiedList

我如何获得原始列表，即

"S Age"- 10
"S Age"- ABC
"S Name "- 30
"S Name "- XYZ

Student s1 = new Student();
s1.setAge(10);
s1.setName("ABC");

Student s2 = new Student();
s2.setAge(30);
s2.setName("XYZ");

ArrayList<Student> al = new ArrayList<Student>();
al.add(s1);
al.add(s2);

// here i am getting list all data
for(int i=0; i<al.size(); i++) {
   System.out.println("F Age " + al.get(i).getAge());
   System.out.println("F Name " + al.get(i).getName());
}

Student s3 = new Student();
s3 = al.get(0);
s3.setAge(50);
s3.setName("Shyam");

for(int i=0;i< al.size(); i++) {
    System.out.println("S Age " + al.get(i).getAge());
    System.out.println("S Name " + al.get(i).getName()); 
}

Answer 1

不是没有很多工作。让学生可以听：

interface Listener {
    public void changed();
}

class Student {
    private List<Listener> listeners = new ArrayList<>();
    public void addListener(Listener l) { listeners.add(l); }

    public void setAge(int age) {
        //...
        listeners.forEach(Listener::changed);
}

并将List实现为一个不允许在项目更改时更新的侦听器：

class MyList<T> implements List<T>, Listener {
    private List<T> delegate = new ArrayList<T>();
    private boolean editable = true;
    public void changed() { editable = false; }
    public void add(T item) {
        if (editable) {
            delegate.add(item);
        } else {
            throw new IllegalStateException(); // or something
        }
    }
    // also delegate all other List methods in the same way
}

我无法真正看到这样做的重点，它确实有一种“怪异的设计”气味。

Answer 2

你必须根据自己的需要改变不同的概念：

您可以使用 unmodifiableList ，这意味着这是一个您无法修改的列表（您无法添加或删除该对象）但您可以确保通过修改其字段来改变其状态......
您可以定义类学生 inmutable，这样您就无法在创建对象后更改其状态...

这些是两种不同的策略，并且不是分离的（您可以在架构中定义一个，两个或不定义。）

Answer 3

为此，您需要使类（Student类）不可修改或不可变，以便您不允许修改对象：

尝试下面的一个：

public final class Student {

    final String name;
    final int age;

    public Student(String name,int age)
    {
        this.name=name;
        this.age =age;
    }
    public String getName()
    {
        return name;
    }
    public int getAge()
    {
        return age;
    }

    public static void main(String[] args) {
        Student s1 = new Student("ABC", 10);

        /*s1.setAge(10);

        s1.setName("ABC");*/


        Student s2 = new Student("XYZ", 30);

        /*s2.setAge(30);

        s2.setName("XYZ");*/

        ArrayList<Student> al = new ArrayList<Student>();

        al.add(s1);

        al.add(s2);

// here i am getting list all data
        for (int i = 0; i < al.size(); i++) {
            System.out.println("F Age " + al.get(i).getAge());
            System.out.println("F Name " + al.get(i).getName());
        }


        Student s3 = new Student("sss",22);
        s3 = al.get(0);


        // Can't modify because it is not allowed
        /*s3.setAge(50);

        s3.setName("Shyam");*/



        for (int i = 0; i < al.size(); i++) {

            System.out.println("S Age " + al.get(i).getAge());

            System.out.println("S Name " + al.get(i).getName());

        }
    }


}

创建不可变类的步骤：

•   Class must be declared as final (So that child classes can’t be created)
•   Data members in the class must be declared as final (So that we can’t change the value of it after object creation)
•   A parameterized constructor
•   Getter method for all the variables in it
•   No setters(To not have option to change the value of the instance variable)

希望这会有所帮助：）

Answer 4

//Very nice question and are very complex but i am able to give answer. please use Map istean of array list because arraylist save Object reference

public static void main(String args[]){
        int age = 20;
        String name = "DS";
        Student s1 = new Student();
        s1.setAge(age);
        s1.setName(name);
        Student s2 = new Student();
        s2.setAge(30);
        s2.setName("PS");

        //List<Student> list=new ArrayList<Student>();  
        Map<Integer,Student> list=new HashMap<Integer,Student>();    
        list.put(1,s1);
        list.put(2,s2);

        Student b =null;
        for(Map.Entry<Integer, Student> entry:list.entrySet()){    
            int key=entry.getKey();  
            b=entry.getValue();  
            System.out.println(key+" Details:");     
            System.out.println(b.getAge()+" "+b.getName());   
        }    
        Student s3 =    new Student();
        int a = b.getAge();
        String a1 = b.getName();
        a = 100;
        a1= "Shyamji";

        for(Map.Entry<Integer, Student> entry:list.entrySet()){    
            int key=entry.getKey();  
            Student b1=entry.getValue();  
            System.out.println(key+" Details:");   
            System.out.println(b1.getAge()+" "+b1.getName());   
        } 
    }

如何在java

4 个答案: