因为你数组定义的时候只有一个下标，长度只是1，值是abcdaabesase。

建议把这一行

String[] str={"abcdaabesase"};

改成

String[] str={"a","b","c","d","a","a","b","e","s","a","s","e"};

全代码

public class HelloWorld {
    public static void main(String[] args) {
		// 定义一个字符串
		String[] str={"a","b","c","d","a","a","b","e","s","a","s","e"};
        int count=0;
        for(int i=0;i<str.length;i++){
            if(str[i]=="a"){
            count++;
            }
        }
        System.out.println("a字母出现"+count+"次"); //好厉害哦，4次了耶~~

    }

}