如题，谢谢了
//: c12:E10_TypedContainer.java
//+M java E10_TypedContainer
/****************** Exercise 10 *****************
* Create a new type of container that uses a
* private ArrayList to hold the objects. Capture
* the type of the first object you put in it,
* and then allow the user to insert objects of
* only that type from then on.
***********************************************/
import java.util.*;
class X {
private String str = "X ";
public String toString() {
return str;
}
}
class Y {
private String str = "Y ";
public String toString() {
return str;
}
}
class Container {
private ArrayList lst= null;
public void add(Object o) {
Class type = null;
if(lst == null) {
type =o.getClass();
lst.add(o);
} else {
if(!type.isInstance(o)) {
System.out.println( "not the same type! ");
return;
}else {
lst.add(o);//这里异常。
}
}
}
public String toString() {
String result = "[ ";
Iterator it = lst.iterator();
while(it.hasNext()) {
result = result + (it.next()).toString() + ", ";
}
return result + "] ";
}
public void cleanup() {
lst.clear();
}
}
public class E10_TypedContainer {
public static void main(String[] args)
throws Exception {
Container tc = new Container();
for(int i = 0;i <8;i++) {
tc.add(new X());//这里异常。
tc.add(new Y());
}
System.out.println(tc);
tc.cleanup();
for(int i = 0;i <8;i++) {
tc.add(new Y());
tc.add(new X());
}
System.out.println(tc);
}
}
异常：
Exception in thread "main " java.lang.NullPointerException
at Container.add(E10_TypedContainer.java:32)
at E10_TypedContainer.main(E10_TypedContainer.java:60)
------解决方案--------------------
----------------------------------------------------
public void add(Object o) {
Class type = null;
if(lst == null) {
type =o.getClass();
lst = new ArrayList <Object> (); //沒有初始化
lst.add(o);
} else {
----------------------------------------------------