ArrayList和ArrayList＜E＞的区别，加不加泛型区别

ArrayList aList1 = new ArrayList();

ArrayList< String > aList2 = new ArrayList<>();

区别在于不设泛型的话，什么类型的变量都能存进去，但是无法遍历

package exam;

import com.sun.deploy.util.StringUtils;

import java.util.ArrayList;

public class Test1 {
    public static void main(String[] args) {
        ArrayList aList1 = new ArrayList();
        aList1.add(1);
        aList1.add(true);
        aList1.add("str");


        
        // String join1 = StringUtils.join(aList1, "~");
        // System.out.println("join1 = " + join1);

        for (int i = 0; i < aList1.size(); i++) {
            System.out.println(aList1.get(i));
        }

        System.out.println("");
        ArrayList aList2 = new ArrayList<>();
        aList2.add("str1");
        aList2.add("2");
        aList2.add("true");

        String join = StringUtils.join(aList2, "-");
        System.out.println("join = " + join);
        System.out.println("");
        for (String str:aList2) {
            System.out.println("str = " + str);
        }
    }
}

改写一下，集合保存Integer类型

package exam;

import com.sun.deploy.util.StringUtils;

import java.util.ArrayList;
import java.util.Collections;

public class Test1 {
    public static void main(String[] args) {
        ArrayList aList1 = new ArrayList();
        aList1.add(11);
        aList1.add(12);
        aList1.add(14);

        //Integer转化为String
        String join1 = StringUtils.join(Collections.singleton(aList1.toString()), "~");
        System.out.println("join1 = " + join1);
        System.out.println("");

        for (int i = 0; i < aList1.size(); i++) {
            System.out.println(aList1.get(i));
        }
    }
}