Java 8 之前如何将 ArrayList 转换为 HashMap?
ArrayList 是一个可调整大小的数组。它位于Java包Java.util 下。它在Java中提供动态数组。 ArrayList 在需要对数组进行大量更改但也比标准数组慢的程序中很有用。 ArrayList 中的元素可以在需要时添加和删除。
HashMap 是一种以“键/值”对形式存储项目的数据结构。它们可以通过另一种类型的索引(如字符串)进行访问。 HashMap 表示为 HashMap
现在为了将 ArrayList 转换为 HashMap,两个用例也被认为是为了说明覆盖概念
- 初始 ArrayList 没有重复项
- 初始 ArrayList 具有重复值。
案例 1:在Java 8 之前,将 ArrayList 转换为 HashMap 的最理想方法是使用增强的 for 循环迭代给定的 ArrayList,并将 String 作为键及其长度作为值插入 HashMap。在这里,ArrayList 的每个元素都可以转换成一个键值对并存储在 HashMap 中。此方法还处理 ArrayList 中的任何重复项,因为插入的条目会在重复键的情况下覆盖值。在这种情况下,不会抛出任何错误或异常。
示例:初始 ArrayList 没有重复项
Java
// Program to convert ArrayList
// to Hashmap before Java 8
// Name of ArrayList chosen randomly: vehicleList
// Importing java generic libraries
import java.util.*;
public class GFG {
// Main driver method
public static void main(String[] args)
{
// Creating ArrayList
ArrayList vehicleList
= new ArrayList<>(Arrays.asList(
"Car", "Bike", "Bus", "Cycle", "Rickshaw"));
// Display message to enter ArrayList
System.out.println("ArrayList: \n");
// Printing ArrayList
for (String vehicle : vehicleList) {
System.out.println(vehicle);
}
// Display message to separate above
// list from hashmap for readability
System.out.println("\nHashMap: \n");
// Creating Hashmap
HashMap vehicleMap
= convertArrayListToHashMap(vehicleList);
for (Map.Entry entry :
vehicleMap.entrySet()) {
System.out.println(entry.getKey() + " : "
+ entry.getValue());
}
}
// Converting ArrayList to HashMap
private static HashMap
convertArrayListToHashMap(ArrayList arrayList)
{
HashMap hashMap = new HashMap<>();
// For-each loop for iteration
for (String str : arrayList) {
hashMap.put(str, str.length());
}
// Returning converted Hashmap
return hashMap;
}
}
Java
// Program to convert ArrayList
// to Hashmap before Java 8
// Importing java generic libraries
import java.util.*;
public class GFG {
// Main driver method
public static void main(String[] args)
{
// Creating ArrayList
ArrayList vehicleList = new ArrayList<>(
// Presence of Duplicate Elements
Arrays.asList("Car", "Bike", "Bus", "Cycle",
"Rickshaw", "Car", "Bike"));
System.out.println("ArrayList: \n");
// Printing ArrayList
for (String vehicle : vehicleList) {
System.out.println(vehicle);
}
// Display message just after ArratList is printed
// for better readability
System.out.println("\nHashMap: \n");
// Creating HashMap
HashMap vehicleMap
= convertArrayListToHashMap(vehicleList);
// Printing above HashMap
for (Map.Entry entry :
vehicleMap.entrySet()) {
System.out.println(entry.getKey() + " : "
+ entry.getValue());
}
}
// Converting ArrayList to HashMap
private static HashMap
convertArrayListToHashMap(ArrayList arrayList)
{
HashMap hashMap = new HashMap<>();
// For-each loop for iteration
for (String str : arrayList) {
hashMap.put(str, str.length());
}
// Returning converted HashMap
return hashMap;
}
}
输出
ArrayList:
Car
Bike
Bus
Cycle
Rickshaw
HashMap:
Bus : 3
Rickshaw : 8
Car : 3
Bike : 4
Cycle : 5
情况 2:初始 ArrayList 具有重复值,由于 HashMap 中的重复键,它会覆盖该值。
Java
// Program to convert ArrayList
// to Hashmap before Java 8
// Importing java generic libraries
import java.util.*;
public class GFG {
// Main driver method
public static void main(String[] args)
{
// Creating ArrayList
ArrayList vehicleList = new ArrayList<>(
// Presence of Duplicate Elements
Arrays.asList("Car", "Bike", "Bus", "Cycle",
"Rickshaw", "Car", "Bike"));
System.out.println("ArrayList: \n");
// Printing ArrayList
for (String vehicle : vehicleList) {
System.out.println(vehicle);
}
// Display message just after ArratList is printed
// for better readability
System.out.println("\nHashMap: \n");
// Creating HashMap
HashMap vehicleMap
= convertArrayListToHashMap(vehicleList);
// Printing above HashMap
for (Map.Entry entry :
vehicleMap.entrySet()) {
System.out.println(entry.getKey() + " : "
+ entry.getValue());
}
}
// Converting ArrayList to HashMap
private static HashMap
convertArrayListToHashMap(ArrayList arrayList)
{
HashMap hashMap = new HashMap<>();
// For-each loop for iteration
for (String str : arrayList) {
hashMap.put(str, str.length());
}
// Returning converted HashMap
return hashMap;
}
}
输出
ArrayList:
Car
Bike
Bus
Cycle
Rickshaw
Car
Bike
HashMap:
Bus : 3
Rickshaw : 8
Car : 3
Bike : 4
Cycle : 5