手撕switch处理String底层源码

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {

	private final char[] value;
    private int hash; //hash值
    
    public String(String original) {
        this.value = original.value;//['a','b','c']的值赋值过去
        this.hash = original.hash;//['a','b','c']的hash值赋值过去
    }
    
    public int hashCode() {
        int h = hash;
        if (h == 0 && value.length > 0) {
           
            char val[] = value; //['a','b','c']

            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i];//hash算法
            }
            hash = h;//96354
        }
        return h;
    }
    
}

package com.qf.switch_string;

import java.io.PrintStream;

public class Test01{
    
	public static void main(String args[]){
        
		label0:{
			
			String str = "Aa";
			String s;//临时的字符串
            //本质获取的str的hash值
			switch ((s = str).hashCode()){//2112
                    
			default:
				break;

			case 2112: //Aa和BB的hash值
				if (!s.equals("Aa")){
					if (s.equals("BB")){
						System.out.println("BB");
						break label0;
					}
				} else{
					System.out.println("Aa");
					break label0;
				}
				break;

			case 119193: 
				if (!s.equals("xyz"))
					break;
				System.out.println("xyz");
				break label0;
			}
			System.out.println("默认的");
		}
	}
}

public static void main(String[] args) {
		//注意：两个不同的字符串hash值有可能相同
		System.out.println("Aa".hashCode());//2112
		System.out.println("BB".hashCode());//2112

		String str = "Aa";


		switch (str) {
		case "abc":
			System.out.println("abc");
			break;
		case "def":
			System.out.println("def");
			break;
		case "xyz":
			System.out.println("xyz");
			break;

		default:
			System.out.println("默认的");
			break;
		}

	}

总结：

底层处理逻辑：先判断hash值，再比较字符串的内容(equals) 因为两个字符串的hash值有可能相同，所以需要比较内容