Clone Java List.

-ArrayList is Cloneable (same for Set and Map), but List is not Cloneable.

-I want to clone what was passed as a List instead of an ArrayList, but there are situations where I can't do it obediently. What should i do?

-The sample is shown below, but there is a method to get the clone method of the actual object class and invoke it.

・ This is where I feel frustrated when using Java. It is the same regret that int ⇔ Integer can be assigned but int [] ⇔ Integer [] cannot be assigned.

-For example, if you do a keySet with HashSet, HashMap $ KeySet will be returned, but this is not Cloneable and cannot be cloned by the method shown in the sample. This kind of thing is a problem ....

・ Another snake leg. The sample uses getDeclaredMethod. -GetDeclaredMethod (getDeclaredMethods) gets the methods declared in the class. -GetMethod (getMethods) including the methods declared in the base class.

★ Addition ★ As pointed out by felis below, you can use CopyOf of List from Java 10. The test program below is running on JDK 1.6.

package jp.avaj.lib.algo;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.List;

import jp.avaj.lib.test.ArTest;
import jp.avaj.lib.test.L;

/**
Clone List

ArrayList is cloneable, but List is not cloneable.Same for Set and Map.
 */
public class Tips0056 {
  public static void main(String[] args) throws NoSuchMethodException, SecurityException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {

    List<String> list = ArList.construct("a,b,c");
    List<String> newList;

    //This is a compile error
    //newList = (List<String>)list.clone();
    L.p("You can do this"); //However, it is unknown whether it is an ArrayList when passed as a List..
    newList = (List<String>)((ArrayList)list).clone();
    L.p(newList.toString());

    L.p("Get the clone method from the actual class and invoke it");
    //Get the clone method in the actual class.If there is a parameter, specify it at the end, but it is unnecessary now
    Method cloneMethod = list.getClass().getDeclaredMethod("clone");
    //If there is no method, an exception will be thrown, so if you come here, there is a method.

    //Execute the method.If there is a parameter, specify it at the end, but it is unnecessary now.
    newList = (List<String>)cloneMethod.invoke(list);

    //Check the result.
    ArTest.equals("clone","expected",3,"size",newList.size());
    ArTest.equals("clone","expected","a","(0)",newList.get(0));
    ArTest.equals("clone","expected","b","(1)",newList.get(1));
    ArTest.equals("clone","expected","c","(2)",newList.get(2));
    //Check visually
    L.p(newList.toString());
  }
}

The result is as follows

You can do this
[a, b, c]
Get the clone method from the actual class and invoke it
OK clone:expected=3:size=3
OK clone:expected=a:(0)=a
OK clone:expected=b:(1)=b
OK clone:expected=c:(2)=c
[a, b, c]