将特定元素从arraylist复制到java中的另一个arraylist [英] Copying specific element from arraylist to another arraylist in java

问题描述

尝试将具有特定值的一个元素（对象）从对象的ArrayList复制到另一个ArrayList

例如：

如何复制具有id的对象1从汽车到bestcars arraylist

trying to copy one element(object) with a specific value from ArrayList of objects to another ArrayList
for example:
How can i copy object that has the id of 1 from cars to bestcars arraylist

Car c1 = new Car("Toyota",1);
Car c2 = new Car("Mercedes",2);

ArrayList<Car>cars = new ArrayList<>();
ArrayList<Car>bestCars = new ArrayList<>();
cars.add(c1);
cars.add(c2);

我的尝试：

What I have tried:

import java.util.ArrayList;
public class Car {

    public String name;
    public int id;

    public Car(String name, int id) {
        this.name = name;
        this.id = id;
    }


    public static void main(String[] args) {

        Car c1 = new Car("Toyota",1);
        Car c2 = new Car("Mercedes",2);

        ArrayList<Car>cars = new ArrayList<>();

        cars.add(c1);
        cars.add(c2);

        ArrayList<Car>bestCars = new ArrayList<>();
        Car p = cars.get(1);
        bestCars.add(p);

        System.out.println("------ Cars -------");
        for(Car test: cars){
            if(test instanceof Car)
            {   System.out.println("---------------------------------------");

                System.out.println("\t\tCar Name:" + test.name +
                        "\nCar ID:"+
                        test.id  );}
        }
        System.out.println("------ bestCars -------");
        for(Car test: cars){
            if(test instanceof Car)
            {   System.out.println("---------------------------------------");

                System.out.println("Car Name:" + test.name +
                        "\nCar ID:"+
                        test.id  );}
        }



    }

}

推荐答案

你可以这样做：

You can do this:

Car c1 = new Car("Toyota",1);
Car c2 = new Car("Mercedes",2);

ArrayList<Car>cars = new ArrayList<>();
ArrayList<Car>bestCars = new ArrayList<>();
cars.add(c1);
cars.add(c2);
            
Car c = cars.stream()
            .filter(x -> x.id == 1)
            .findFirst()
            .get();
bestCars.add(c);
System.out.println(bestCars.get(0).name); // Toyota

如何运作：

• cars.stream（）创建一个 Stream 来自 ArrayList 。在这个对象上，你可以使用过滤器函数。
• .filter（x - > x.id = = 1）将 1 的所有 Car 作为 > id （在这个例子中有一个这样的汽车）。 x - > x.id == 1 是一个 lambda表达式（这是一个可以用这样的简单语句创建的函数，不必属于某个类）。表达式有一个参数 x 并返回 x.id == 1 ，一个布尔值。
• .findFirst（）返回可选< Car> [ ^ ] - 可选是一个容器，可能包含也可能不包含非空对象。如果 filter 结果是过滤掉所有对象而没有返回单个对象，则Optional将为空（但在此示例中不是这种情况）。
• .get（）从 Optional Car >。请注意，如果Optional中没有值，则会抛出 NoSuchElementException 。在此示例中，很明显存在匹配值。在您不是100％确定具有值的Optional的应用程序中，请使用 isPresent 。


How this works:




• cars.stream() creates a Stream from the ArrayList. On this object, there is the filter function you can use.
• .filter(x -> x.id == 1) will take all Cars that have 1 as id (there is one Car like that, in this example). x -> x.id == 1 is a lambda expression (which is a function that can be created with a simple statement like this, without having to belong to a class). The expression has one parameter x and returns x.id == 1, a Boolean.
• .findFirst() returns an Optional<Car>[^] - an Optional is a container which may, or may not, contain a non-null object. The Optional would be empty if filter turned out to filter out all objects and not returning a single one (but that is not the case in this example).
• .get() extracts the Car from the Optional. Note that, if no value is present in the Optional, this will throw a NoSuchElementException. In this example, it's clear that there is a matching value. In applications where you are not 100% certain of an Optional to have a value, use isPresent.




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