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Java SE 047 Realize your own Comparator and Collections detailed
Article Directory
1. Realize your own Comparator
1.1 Example 1: Comparison of characters
package com.javase.treeset;
import java.util.Comparator;
import java.util.Iterator;
import java.util.TreeSet;
public class TreeSetTest3 {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
TreeSet set = new TreeSet(new MyComparator());
set.add("C");
set.add("F");
set.add("A");
set.add("D");
set.add("E");
set.add("B");
for(Iterator iter = set.iterator(); iter.hasNext();){
String value = (String)iter.next();
System.out.println(value);
}
}
}
@SuppressWarnings("unchecked")
class MyComparator implements Comparator{
//如果这个方法返回一个-1代表o1比o2小，对于字符串儿来说就是o1在o2的前面。
//如果等于0，则o1与o2是相等的。
//如果>0是正数，则o1>o2
//实际上是按照升序的方式去考虑的。
//现在需要实现字符串儿的降序排序可用s2.compareTo(s1);
@Override
public int compare(Object o1, Object o2) {
//强制转换成放进集合中元素的类型
String s1 = (String)o1;
String s2 = (String)o2;
return s2.compareTo(s1);
}
}
package com.javareview.collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.TreeSet;
public class TreeSetTest {
public static void main(String[] args) {
TreeSet<String> set = new TreeSet<String>(new MyComparator());
set.add("C");
set.add("F");
set.add("A");
set.add("D");
set.add("E");
set.add("B");
for(Iterator<String> iter = set.iterator(); iter.hasNext();){
String value = (String)iter.next();
System.out.println(value);
}
}
}
class MyComparator implements Comparator<String>{
@Override
public int compare(String o1, String o2) {
return o1.compareTo(o2);
}
}
1.2 Example 2: Comparison of integers
package com.javase.treeset;
import java.util.Comparator;
import java.util.Iterator;
import java.util.TreeSet;
public class TreeSetTest2 {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
TreeSet set = new TreeSet(new MyComparator2());
Person p1 = new Person(10);
Person p2 = new Person(20);
Person p3 = new Person(30);
Person p4 = new Person(40);
Person p5 = new Person(50);
set.add(p1);
set.add(p2);
set.add(p3);
set.add(p4);
set.add(p5);
for(Iterator iter = set.iterator(); iter.hasNext();){
System.out.println(((Person)iter.next()).score);
}
}
}
class Person{
int score;
public Person(int score){
this.score = score;
}
@Override
public String toString() {
return String.valueOf(score);
}
}
@SuppressWarnings("unchecked")
class MyComparator2 implements Comparator{
//比较器接收的参数Object o1与o2是由放进集合中的元素来决定的，此例中放进集合中的元素是Person，则在进行比较时需要将传进集合中的Object强制转换成Person之后然后再进行比较。
@Override
public int compare(Object o1, Object o2) {
Person p1 = (Person) o1;
Person p2 = (Person) o2;
return p1.score - p2.score;
}
}
2. Collections of auxiliary functions Collections
2.1 Example 1: Reverse sorting of set elements
package com.javase.collections;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.LinkedList;
public class CollectionsTest {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
LinkedList li = new LinkedList();
li.add(new Integer(-10));
li.add(new Integer(15));
li.add(new Integer(12));
li.add(new Integer(-5));
Comparator comparator = Collections.reverseOrder();
//第一个参数是要排序的集合，第二个参数是排序规则
Collections.sort(li,comparator);
for(Iterator iter = li.iterator();iter.hasNext();){
System.out.println((Integer)iter.next());
}
}
}
2.2 Example 2: Shuffle the set (shuffle(List list))
package com.javase.collections;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.LinkedList;
public class CollectionsTest {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
LinkedList li = new LinkedList();
li.add(new Integer(-10));
li.add(new Integer(15));
li.add(new Integer(12));
li.add(new Integer(-5));
Comparator comparator = Collections.reverseOrder();
//第一个参数是要排序的集合，第二个参数是排序规则
Collections.sort(li,comparator);
for(Iterator iter = li.iterator();iter.hasNext();){
System.out.println((Integer)iter.next());
}
System.out.println();
//将集合顺序打乱
Collections.shuffle(li);
for(Iterator iter = li.iterator();iter.hasNext();){
System.out.println((Integer)iter.next());
}
}
}
2.3 Example 3: Get the maximum and minimum values in the set
package com.javase.collections;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.LinkedList;
public class CollectionsTest {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
LinkedList li = new LinkedList();
li.add(new Integer(-10));
li.add(new Integer(15));
li.add(new Integer(12));
li.add(new Integer(-5));
Comparator comparator = Collections.reverseOrder();
//第一个参数是要排序的集合，第二个参数是排序规则
Collections.sort(li,comparator);
for(Iterator iter = li.iterator();iter.hasNext();){
System.out.println((Integer)iter.next());
}
System.out.println();
//将集合顺序打乱
Collections.shuffle(li);
for(Iterator iter = li.iterator();iter.hasNext();){
System.out.println((Integer)iter.next());
}
System.out.println();
System.out.println("min value:"+Collections.min(li));
System.out.println("max value:"+Collections.max(li));
}
}