更新

java_demo/src/main/java/com/cmower/java_demo/fanxing/Arraylist.java

@@ -52,4 +52,19 @@ class Arraylist<E> {
         }
         return sb.toString();
     }
+    
+    public int size() {
+        return size;
+    }
+    
+    E elementData(int index) {
+        return (E) elementData[index];
+    }
+    
+    public E set(int index, E element) {
+
+        E oldValue = elementData(index);
+        elementData[index] = element;
+        return oldValue;
+    }
 }
\ No newline at end of file

java_demo/src/main/java/com/cmower/java_demo/fanxing/Cmower.java

 package com.cmower.java_demo.fanxing;
 
+import java.util.Collections;
+
 public class Cmower {
-    
-    public static void main(String[] args) {
-        Arraylist<Wanger> list = new Arraylist<>(4);
-        
-        Wanger wanger = new Wanger();
-        list.add(wanger);
-        
-        Wangxiaoer wangxiaoer = new Wangxiaoer();
-        list.add(wangxiaoer);
-        
-        Arraylist<? extends Wanger> list1 = list;
-        Wanger w = list1.get(1);
-        System.out.println(w);
-        
-        System.out.println(list1.contains(wangxiaoer));
-    }
+
+	public static void main(String[] args) {
+		Arraylist<Wanger> list = new Arraylist<>(4);
+
+		Wanger wanger = new Wanger();
+		list.add(wanger);
+
+		Wangxiaoer wangxiaoer = new Wangxiaoer();
+		list.add(wangxiaoer);
+
+		Arraylist<? extends Wanger> list1 = list;
+		Wanger w = list1.get(1);
+		System.out.println(w);
+
+		System.out.println(list1.contains(wangxiaoer));
+		
+		System.out.println(list1 == list);
+
+		Arraylist<? extends Wanger> list2 = new Arraylist<>(4);
+		list2.add(null);
+		// list2.add(new Wanger());
+		// list2.add(new Wangxiaoer());
+
+		Wanger w2 = list2.get(0);
+		
+Arraylist<? super Wanger> list3 = new Arraylist<>(4);
+list3.add(new Wanger());
+list3.add(new Wangxiaoer());
+
+//Collections
+
+//		Wanger w3 = list3.get(0);
+		
+		
+	}
 
 }

java_demo/src/main/java/com/cmower/java_demo/fanxing/Collections.java

+package com.cmower.java_demo.fanxing;
+
+public class Collections {
+	public static <T> void copy(Arraylist<? super T> dest, Arraylist<? extends T> src) {
+		for (int i = 0; i < src.size(); i++)
+			dest.set(i, src.get(i));
+	}
+}