JAVA#TreeSet'学习札记

1.TreeSet 是 Set 接口的实现类，TreeSet 可以确保集合元素处于排序状态；

2.TreeSet有两种排序方法——自然排序和自定义排序，默认情况下采用自然排序；

3.自然排序：

1️⃣TreeSet 会调用集合元素的 compareTo(Object obj) 方法来比较元素之间的大小关系，然后将集合元素按升序排列

2️⃣如果试图把一个对象添加到 TreeSet 时，则该对象的类必须实现 Comparable 接口。

3️⃣实现 Comparable 的类内必须实现 compareTo(Object obj) 方法，两个对象即通过 compareTo(Object obj) 方法的返回值来比较大小。

4️⃣向TreeSet内添加的对象应是同属一个类的；

5️⃣

public class Setree {
    @Test
    public void MM() {
        Set setree = new TreeSet();

        setree.add(new Beauty(12,"PG One"));
        setree.add(new Beauty(23,"老流氓美美美"));
        setree.add(new Beauty(24,"我科银河系第一美"));
        setree.add(new Beauty(45,"皮二爷还是很美"));
        setree.add(new Beauty(278,"科比为什么这么美"));
        setree.add(new Beauty(2345,"Kobe一报由"));
        Iterator iterator=setree.iterator();
        while(iterator.hasNext()){
            System.out.println(iterator.next());
        }

    }
}
class Beauty implements Comparable{
    private Integer BeautyNum;
    private String name;

    public Beauty(Integer BeautyNum, String name) {
        this.BeautyNum = BeautyNum;
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getBeautyNum() {
        return BeautyNum;
    }

    public void setBeautyNum(Integer beautyNum) {
        BeautyNum = beautyNum;
    }

    @Override
    public String toString() {
        return "Name[name = " + this.getName() + " BeautyNumber[Num = " + this.getBeautyNum()+"]";
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Beauty mm = (Beauty)obj;
        if (name == null) {
            if (mm.name != null)
                return false;
        } else if (!name.equals(mm.name))
            return false;
        if (BeautyNum != mm.BeautyNum)
            return false;

        return true;
    }


    @Override
    public int hashCode() {
        final int kobe = 31;
        int iverson = 3;
        iverson = kobe * iverson + BeautyNum;
        iverson = kobe * iverson + ((name == null) ? 0 : name.hashCode());
        return iverson;
    }

    @Override
    public int compareTo(Object o) {
        if(o instanceof Beauty){
            Beauty mm=(Beauty)o;
            int num=this.getBeautyNum().compareTo(mm.getBeautyNum());
            if(num==0){
                return this.getName().compareTo(mm.getName());
            }
            else
                return num;
        }
        return 0;
    }
}

Name[name = PG One] BeautyNumber[Num = 12]
Name[name = 老流氓美美美] BeautyNumber[Num = 23]
Name[name = 我科银河系第一美] BeautyNumber[Num = 24]
Name[name = 皮二爷还是很美] BeautyNumber[Num = 45]
Name[name = 科比为什么这么美] BeautyNumber[Num = 278]
Name[name = Kobe一报由] BeautyNumber[Num = 2345]

4.自定义排序

1️⃣自定义排序即为让对象根据自己的特殊需求进行定制的排序操作；

2️⃣通过Comparator接口，重写其中的compare方法进行对象的比较，再将Comparator实例化的对象传入TreeSet中即可；

3️⃣

public class Treset {
    @Test
    public void MM(){
        Comparator cp=new Comparator() {
            @Override
            public int compare(Object o1, Object o2) {
                if(o1 instanceof Beauty&&o2 instanceof Beauty){
                    Beauty b1=(Beauty)o1;
                    Beauty b2=(Beauty)o2;
                    int i=b1.getBeautyNum().compareTo(b2.getBeautyNum());
                    if(i==0){
                        return b1.getName().compareTo(b2.getName());
                    }
                    return i;
                }
                return 0;
            }
        };
        Set st=new TreeSet(cp);
        st.add(new Beauty(12,"PG One"));
        st.add(new Beauty(23,"老流氓美美美"));
        st.add(new Beauty(24,"我科银河系第一美"));
        st.add(new Beauty(45,"皮二爷还是很美"));
        st.add(new Beauty(278,"科比为什么这么美"));
        st.add(new Beauty(2345,"Kobe一报由"));
        st.add(new Beauty(1,"妈妈最美"));
        Iterator iterator=st.iterator();
        while(iterator.hasNext()){
            System.out.println(iterator.next());
        }

    }
}
class Beauty implements Comparable{
    private Integer BeautyNum;
    private String name;

    public Beauty(Integer BeautyNum, String name) {
        this.BeautyNum = BeautyNum;
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getBeautyNum() {
        return BeautyNum;
    }

    public void setBeautyNum(Integer beautyNum) {
        BeautyNum = beautyNum;
    }

    @Override
    public String toString() {
        return "Name[name = " + this.getName() + "] BeautyNumber[Num = " + this.getBeautyNum()+"]";
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Beauty mm = (Beauty)obj;
        if (name == null) {
            if (mm.name != null)
                return false;
        } else if (!name.equals(mm.name))
            return false;
        if (BeautyNum != mm.BeautyNum)
            return false;

        return true;
    }


    @Override
    public int hashCode() {
        final int kobe = 31;
        int iverson = 3;
        iverson = kobe * iverson + BeautyNum;
        iverson = kobe * iverson + ((name == null) ? 0 : name.hashCode());
        return iverson;
    }

    @Override
    public int compareTo(Object o) {
        if(o instanceof Beauty){
            Beauty mm=(Beauty)o;
            int num=this.getBeautyNum().compareTo(mm.getBeautyNum());
            if(num==0){
                return this.getName().compareTo(mm.getName());
            }
            else
                return num;
        }
        return 0;
    }
}

Name[name = 妈妈最美] BeautyNumber[Num = 1]
Name[name = PG One] BeautyNumber[Num = 12]
Name[name = 老流氓美美美] BeautyNumber[Num = 23]
Name[name = 我科银河系第一美] BeautyNumber[Num = 24]
Name[name = 皮二爷还是很美] BeautyNumber[Num = 45]
Name[name = 科比为什么这么美] BeautyNumber[Num = 278]
Name[name = Kobe一报由] BeautyNumber[Num = 2345]