學習筆記 | Java的List、Set、Map集合

Java集合

01 List：動態數組(元素個數未知)

package example;


import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class ListDemo {

	public static void main(String[] args) {
		List<String> list = new ArrayList<>();
		// 判斷是否爲空
		System.out.println(list.isEmpty()? "list is empty." : "list is not empty.");
		// 添加元素
		list.add("World! ");
		list.add(0,"Hello ");// 通過指定下標添加
		list.add("I ");
		list.add("am ");
		list.add("learning ");
		list.add("Java ");
		System.out.println(list);// 打印list
		
		// 通過下標訪問
		for(int i =0;i< list.size() ; i ++) {
			System.out.print(list.get(i)+ "、");
		}
		System.out.println();

		// 通過Iterator循環
		Iterator<String> iterator = list.iterator();
		while(iterator.hasNext()) {
			System.out.print(iterator.next() + "、");
		}
		System.out.println();
		// 通過For-Each遍歷
		for(String item:list) {
			System.out.print(item+"、");
		}
		System.out.println();
		
		System.out.println(list.isEmpty()? "list is empty." : "list is not empty.");
		System.out.println("list長度："+list.size());
		System.out.println("更新前，下標爲3的元素是："+ list.get(3));
		list.set(3, "do not ");// 更新下標爲3的值；
		System.out.println("更新後，下標爲3的元素是："+ list.get(3));
		
		List<String> sub = list.subList(2, 5);
		System.out.println("子列表sub的長度是：" + sub.size());
	}

}

輸出：

list is empty.
[Hello , World! , I , am , learning , Java ]
Hello 、World! 、I 、am 、learning 、Java 、
Hello 、World! 、I 、am 、learning 、Java 、
Hello 、World! 、I 、am 、learning 、Java 、
list is not empty.
list長度：6
更新前，下標爲3的元素是：am 
更新後，下標爲3的元素是：do not 
子列表sub的長度是：3

02 Set：不含重複元素

package example;

import java.util.Set;
import java.util.HashSet;
import java.util.Iterator;

public class SetDemo {

	public static void main(String[] args) {
		Set<String> sets = new HashSet<String>();
		// 判斷是否爲空
		System.out.println(sets.isEmpty()? "Set is empty." : "Set is not empty.");
		// 添加元素
		sets.add("A");
		sets.add("D");
		sets.add("B");
		sets.add("C");
		sets.add("A");// 添加重複元素
		sets.add("B");// 添加重複元素
		
		// for each 遍歷
		for(String item:sets) {
			System.out.print(item+"、");
		}
		System.out.println();
		// Iterator遍歷
		Iterator<String> iterator = sets.iterator();
		
		while(iterator.hasNext()) {
			System.out.print(iterator.next()+"、");
		}
		System.out.println();
		System.out.println(sets);// 打印sets
		System.out.println(sets.isEmpty()? "Set is empty." : "Set is not empty.");
		System.out.println("Set 的個數：" + sets.size());		
		System.out.println( "Set "+ (sets.contains("C") ? "" : "不" )+ "包含 C！" );
		System.out.println( "Set "+ (sets.contains("E") ? "" : "不" )+ "包含 E！" );
		sets.clear();// 清空sets
		System.out.println(sets.isEmpty()? "Set is empty." : "Set is not empty.");
	}

}

輸出：

Set is empty.
A、B、C、D、
A、B、C、D、
[A, B, C, D]
Set is not empty.
Set 的個數：4
Set 包含 C！
Set 不包含 E！
Set is empty.

03 Map：鍵值對的集合

package example;

import java.util.Map;
import java.util.HashMap;

public class MapDemo {

	public static void main(String[] args) {
		Map<Integer,String> maps = new HashMap<>();
		// 判斷是否爲空
		System.out.println(maps.isEmpty()? "Map is empty." : "Map is not empty.");
		// 添加元素
		maps.put(1,"A");
		maps.put(2,"B");
		maps.put(3,"C");
		maps.put(4,"D");
		System.out.println(maps);// 打印maps
		
		maps.put(1,"E");// 更新 鍵爲1的值
		System.out.println(maps);// 打印maps
		System.out.println(maps.isEmpty()? "Map is empty." : "Map is not empty.");
		System.out.println("Map 的個數：" + maps.size());		
		System.out.println( "Map "+ (maps.containsKey(3) ? "" : "不" )+ "包含鍵3！" );
		System.out.println( "Map "+ (maps.containsValue("E") ? "" : "不" )+ "包含 值E！" );
		System.out.println("鍵2對應的值是： "+ maps.get(2));// 獲取鍵對應的值
		maps.clear();// 清空maps
		System.out.println(maps.isEmpty()? "Map is empty." : "Map is not empty.");
		System.out.println(maps.get(2));
	}
}

輸出：

Map is empty.
{1=A, 2=B, 3=C, 4=D}
{1=E, 2=B, 3=C, 4=D}
Map is not empty.
Map 的個數：4
Map 包含鍵3！
Map 包含 值E！
鍵2對應的值是： B
Map is empty.
null

map的遍歷

package example;

import java.util.Map;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Set;

public class MapDemo2 {

	public static void main(String[] args) {
		Map<Integer,String> maps = new HashMap<>();
		// 添加元素
		maps.put(1,"A");
		maps.put(2,"B");
		maps.put(3,"C");
		maps.put(4,"D");
		maps.put(1,"E");// 更新 鍵爲1的值
		
		// 通過keyset來遍歷
		Set<Integer> keys = maps.keySet();
		for(Integer key : keys) {
			System.out.println("key: " + key +" ->  value:"+maps.get(key) );
		}
		System.out.println("------------------");
	
		// 通過 entrySet來遍歷
		Set<Map.Entry<Integer, String>> keyValues = maps.entrySet();
		for(Map.Entry<Integer, String> kv : keyValues) {
			System.out.println("key: " + kv.getKey() +" ->  value:"+kv.getValue() );
		}
		
		System.out.println("------------------");
		
		// 通過Iterator來遍歷
		// TODO 由於maps.entrySet是一個Set，所以可以參考Set的遍歷方法，使用Iterator來遍歷
		
		Iterator<Map.Entry<Integer, String>> iterator = maps.entrySet().iterator();
		while(iterator.hasNext()) {
			Map.Entry<Integer, String> kv = iterator.next();
			System.out.println("key: " + kv.getKey() +" ->  value:"+kv.getValue() );
		}
	}

}

輸出：

key: 1 ->  value:E
key: 2 ->  value:B
key: 3 ->  value:C
key: 4 ->  value:D
------------------
key: 1 ->  value:E
key: 2 ->  value:B
key: 3 ->  value:C
key: 4 ->  value:D
------------------
key: 1 ->  value:E
key: 2 ->  value:B
key: 3 ->  value:C
key: 4 ->  value:D