java 添加一组元素

在java包中的Arrays和Collection类中都有很多实用方法,可以在一个Collection中添加一组元素,Array.asList()方法接受一个数组或是一个用逗号分隔的元素列表(使用可变参数),并将其转化为一个List对象,Collections.addAll()方法接受一个Collection对象,以及一个数组或一个逗号分割的列表,将元素添加到Collection中

//: holding/AddingGroups.java
// Adding groups of elements to Collection objects.
 package object;
import java.util.*;

public class AddingGroups {
  public static void main(String[] args) {
    Collection<Integer> collection =
      new ArrayList<Integer>(Arrays.asList(1, 2, 3, 4, 5));//Collection可以接受另一个Collection初始化自己
    Integer[] moreInts = { 6, 7, 8, 9, 10 };
    collection.addAll(Arrays.asList(moreInts));//collection.addAll运行速度快
    // Runs significantly faster, but you can't
    // construct a Collection this way:collection.addAll只能接受另一个collection作为参数
    Collections.addAll(collection, 11, 12, 13, 14, 15);
    Collections.addAll(collection, moreInts);//将moreInts添加进collection
    // Produces a list "backed by" an array:
    List<Integer> list = Arrays.asList(16, 17, 18, 19, 20);//list底层表示是数组,不能调整数组大小
    list.set(1, 99); // OK -- modify an element
    //list.add(21); // Runtime error because the
                     // underlying array cannot be resized.
  }
} ///:~

Arrays.asList()方法不能之间向上转型,必须插入一条线索,以告诉编译器队医Arrays.asList()产生的List类型,实际的目标是什么这称为: 显示类型说明参数说明

Map除了用另一个Map之外,Java标准类库没有提供其他任何自动初始化它们的方法

//: holding/AsListInference.java
// Arrays.asList() makes its best guess about type.
 package object;
import java.util.*;

class Snow {}
class Powder extends Snow {}
class Light extends Powder {}
class Heavy extends Powder {}
class Crusty extends Snow {}
class Slush extends Snow {}

public class AsListInference {
  public static void main(String[] args) {
    List<Snow> snow1 = Arrays.asList(
      new Crusty(), new Slush(), new Powder());
    // Won't compile:
    // List<Snow> snow2 = Arrays.asList(
    //   new Light(), new Heavy()); //Arrays.asList会创建List<Powder>而不是,,List<Snow>
    // Compiler says:
    // found   : java.util.List<Powder>
    // required: java.util.List<Snow>

    // Collections.addAll() doesn't get confused:
    List<Snow> snow3 = new ArrayList<Snow>();
    Collections.addAll(snow3, new Light(), new Heavy());
    //Collections.addAll 从它的第一个参数中了解到了目标类型是什么
    // Give a hint using an
    // explicit type argument specification:
    List<Snow> snow4 = Arrays.<Snow>asList(
       new Light(), new Heavy());
  }
} ///:~