java - 我可以请人验证我的 SCJP 考试 Collection 吗

Question

我一直在准备 SCJP，现在是 Oracle 认证专业 Java SE 程序员考试。

我很难理解所有不同的集合以及何时使用它们。我也喜欢闪存卡。因此，我尝试创建一组本质上相同的类，除了它们使用的集合之外。我必须确定输出的结果以及每个集合的主要“特征”是什么。

不幸的是我不相信自己。我希望有人确认所有信息是否准确或是否有遗漏。经过一些反馈/更正后，我认为这对于其他试图理解 Java 集合的人来说将是一个很好的练习。

涵盖的集合有:HashMap、Hashtable、TreeMap、LinkedHashMap、HashSet、TreeSet、LinkedHashSet、ArrayList、Vector、LinkedList、PriorityQueue。

我也把所有文件都分开了，可以在这里下载:http://www.allgo.com/personal/MyCollections.zip

提前致谢

import java.util.*;import java.lang.*;class MyItem implements Comparable{    private String name;    MyItem(String n){ name = n; }    public String toString(){return name;}    public String getName(){return name;}    public boolean equals(Object obj){        if(this==obj) return true;        else if(obj==null) return false;        else if(getName() != ((MyItem)obj).getName()) return false;        else return true;    }    public int hashCode(){ return 5; }    public int compareTo(MyItem b){return b.getName().compareTo(getName());}}public class MyCollections{    public static void main(String[] args){        MyHashMap.main(args);           System.out.println("HashMap: Hash=Unsorted, Unordered. Map=key/value pair\n##\n");        MyHashtable.main(args);         System.out.println("Hashtable: Thread Safe. Hash=Unsorted, Unordered. Map=key/value pair\n##\n");        MyTreeMap.main(args);           System.out.println("TreeMap: Tree=sorted. Map=key/value.\n##\n");        MyLinkedHashMap.main(args);     System.out.println("LinkedHashMap: Linked=Maintains Insertion Order. Hash=unsorted, unordered. Map=key/value pair.\n##\n");        MyHashSet.main(args);           System.out.println("HashSet: Hash=Unsorted, Unordered. Set=Unique. Define=equals/hashCode\n##\n");        MyTreeSet.main(args);           System.out.println("TreeSet: Tree=Sorted. Set=Unique. Define=Comparable/Comparator\n##\n");        MyLinkedHashSet.main(args);     System.out.println("LinkedHashSet: Liniked=Maintains Insertion Order. Hash=Unsorted. Set=Unique. Define=equals/hashCode\n##\n");        MyArrayList.main(args);         System.out.println("ArrayList: List=Queue. Maintains insertion order, Allowed duplicates\n##\n");        MyVector.main(args);            System.out.println("Vector: Thread Safe. ArrayList. Maintains Insertion Order, Allows duplicates\n##\n");        MyLinkedList.main(args);        System.out.println("LinkedList: Linked=Maintaines Insertion Order. List=Queue. Advanced ArrayList with more methods.\n##\n");        MyPriorityQueue.main(args);     System.out.println("PriorityQueue: Define=Comparable/comparator\n##\n");    }}class MyHashMap{    public static void main(String[] args){        HashMap c = new HashMap();        MyItem Eight = new MyItem("Eight");        c.put(5, new MyItem("Five")); c.put(1, new MyItem("One")); c.put(8, Eight); c.put(3, new MyItem("Three"));        c.put(4, new MyItem("Four")); c.put(1, new MyItem("1")); c.put(8, Eight); c.put(9, new MyItem("Nine"));        c.remove(3); c.put(7, new MyItem("Seven"));        System.out.println(c);//output?    }}class MyHashtable{    public static void main(String[] args){        Hashtable c = new Hashtable();        MyItem Eight = new MyItem("Eight");        c.put(5, new MyItem("Five")); c.put(1, new MyItem("One")); c.put(8, Eight); c.put(3, new MyItem("Three"));        c.put(4, new MyItem("Four")); c.put(1, new MyItem("1")); c.put(8, Eight); c.put(9, new MyItem("Nine"));        c.remove(3); c.put(7, new MyItem("Seven"));        System.out.println(c);//output?    }}class MyTreeMap{    public static void main(String[] args){        TreeMap c = new TreeMap();        MyItem Eight = new MyItem("Eight");        c.put(5, new MyItem("Five")); c.put(1, new MyItem("One")); c.put(8, Eight); c.put(3, new MyItem("Three"));        c.put(4, new MyItem("Four")); c.put(1, new MyItem("1")); c.put(8, Eight); c.put(9, new MyItem("Nine"));        c.remove(3); c.put(7, new MyItem("Seven"));        System.out.println(c);//output?    }}class MyLinkedHashMap{    public static void main(String[] args){        LinkedHashMap c = new LinkedHashMap();        MyItem Eight = new MyItem("Eight");        c.put(5, new MyItem("Five")); c.put(1, new MyItem("One")); c.put(8, Eight); c.put(3, new MyItem("Three"));        c.put(4, new MyItem("Four")); c.put(1, new MyItem("1")); c.put(8, Eight); c.put(9, new MyItem("Nine"));        c.remove(3); c.put(7, new MyItem("Seven"));        System.out.println(c);//output?    }}class MyHashSet{    public static void main(String[] args){        HashSet c = new HashSet();        MyItem Eight = new MyItem("Eight");        c.add(new MyItem("Five")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Three"));        c.add(new MyItem("Four")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Nine"));        c.remove(3); c.add(new MyItem("Seven"));        System.out.println(c);//output?    }}class MyTreeSet{    public static void main(String[] args){        TreeSet c = new TreeSet();        MyItem Eight = new MyItem("Eight");        c.add(new MyItem("Five")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Three"));        c.add(new MyItem("Four")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Nine"));        c.remove(Eight); c.add(new MyItem("Seven"));        System.out.println(c);//output?    }}class MyLinkedHashSet{    public static void main(String[] args){        LinkedHashSet c = new LinkedHashSet();        MyItem Eight = new MyItem("Eight");        c.add(new MyItem("Five")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Three"));        c.add(new MyItem("Four")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Nine"));        c.remove(3); c.add(new MyItem("Seven"));        System.out.println(c);//output?    }}class MyArrayList{    public static void main(String[] args){        ArrayList c = new ArrayList();        MyItem Eight = new MyItem("Eight");        c.add(new MyItem("Five")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Three"));        c.add(new MyItem("Four")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Nine"));        c.remove(3); c.add(new MyItem("Seven"));        System.out.println(c);//output?    }}class MyVector{    public static void main(String[] args){        Vector c = new Vector();        MyItem Eight = new MyItem("Eight");        c.add(new MyItem("Five")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Three"));        c.add(new MyItem("Four")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Nine"));        c.remove(3); c.add(new MyItem("Seven"));        System.out.println(c);//output?    }}class MyLinkedList{    public static void main(String[] args){        LinkedList c = new LinkedList();        MyItem Eight = new MyItem("Eight");        c.add(new MyItem("Five")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Three"));        c.add(new MyItem("Four")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Nine"));        c.remove(3); c.add(new MyItem("Seven"));        System.out.println(c);//output?    }}class MyPriorityQueue{    public static void main(String[] args){        PriorityQueue c = new PriorityQueue();        MyItem Eight = new MyItem("Eight");        c.offer(new MyItem("Five")); c.offer(new MyItem("One")); c.offer(Eight); c.offer(new MyItem("Three"));        c.offer(new MyItem("Four")); c.offer(new MyItem("One")); c.offer(Eight); c.offer(new MyItem("Nine"));        System.out.println(c.peek());        System.out.println(c.poll());        c.offer(new MyItem("Seven"));        System.out.println(c);//output?    }}

Answer 1

对于初学者，您应该 refactor你的代码。基本上，无论你在哪里使用“复制粘贴”，都不要这样做。

创建一个像这样的方法:

private static void fill(Collection c) {
    MyItem Eight = new MyItem("Eight");
    c.add(new MyItem("Five")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Three"));
    c.add(new MyItem("Four")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Nine"));
    c.remove(3); c.add(new MyItem("Seven"));
    System.out.println(c);//output?
}

然后，不要使用您现有的方法，而是执行以下操作:

class MyVector{
    public static void main(String[] args){
        Vector c = new Vector();
        fill(c);
    }
}

并对您拥有的所有集合执行此操作。

接下来，对您的 map 执行类似的操作:

private static void fill(Map<?,?> map) {
    MyItem Eight = new MyItem("Eight");
    map.put(5, new MyItem("Five")); map.put(1, new MyItem("One")); map.put(8, Eight); map.put(3, new MyItem("Three"));
    map.put(4, new MyItem("Four")); map.put(1, new MyItem("1")); map.put(8, Eight); map.put(9, new MyItem("Nine"));
    map.remove(3); map.put(7, new MyItem("Seven"));
    System.out.println(map);//output?
}

您的代码将会缩小、变得可读，甚至可能有一天变得可用。