以下是代码：
public class Fisher{
private String name;
private Fish [] fishCaught;
private int numFishCaught;
private int keepSize;
public static int LIMIT =10;
private boolean full;
public int getnumFishCaught(){
return numFishCaught;
}
public Fisher(String n,int k){
this.name=n;
this.keepSize=k;
this.fishCaught =new Fish[LIMIT];
}
public String toString(){
return this.name + " with " + numFishCaught+ " fish ";
}
public void keep(Fish f){
if(numFishCaught<LIMIT){
this.fishCaught[numFishCaught]=f;
numFishCaught++;
full=false;
}else{
full=true;
numFishCaught=LIMIT;
}
}
public boolean likes(Fish f1){
if(f1.getSpecies().equals("Sunfish")){
return false;
}else{
if(f1.getSize()>=this.keepSize){
return true;
}else{
return false;
}
}
}
public void listFish(){
System.out.println( this.name +" with "+ numFishCaught + " fish as follow: ");
for(int i=0;i<numFishCaught;i++){
System.out.println("A " + this.fishCaught[i].getSize() +" cm " + this.fishCaught[i].getSpecies());
}
}
public void goFishingIn(Pond p){
if(likes(p.catchAFish())){ // the problem happens here
keep(p.catchAFish());
if(full==true)
p.add(p.catchAFish());
}
}
public void giveAwayFish(Fisher f1,Pond pp){
this.keepSize=f1.keepSize;
for(int i=0;i<numFishCaught;i++){
if(likes(fishCaught[i])==true){
f1.keep(fishCaught[i]);
}else{
pp.add(fishCaught[i]);
}
}
}
}
另外两个班，鱼和池，是：
public class Pond {
private Fish [] fish;
private int numFish;
private int capacity;
private boolean full;
private Fish fff;
public Pond(int c){
capacity=c;
fish= new Fish[capacity];
fff= new Fish(0,"UNKNOWN");
}
public boolean isFull(){
if(numFish>capacity){
full=true;
return true;
}
else{
full=false;
return false;
}
}
public int getNumFish(){
return numFish;
}
public void add(Fish f){
if(full==false){
fish[numFish]=f;
numFish++;
}
}
public void listFish(){
System.out.println("Pond with " + numFish + " fish as follow: ");
for(int i=0;i<numFish;i++){
System.out.println("A " + fish[i].getSize() +" cm " + fish[i].getSpecies());
}
}
public Fish catchAFish(){
int num= (int)(Math.random() * (numFish-1));
if(fish[num]!=null && numFish>0){
fff= fish[num];
fish[num]= fish[numFish-1];
fish[numFish-1]=null;
numFish--;
return fff;
}
else
return null;
}
public String toString(){
return "Pond with "+ numFish +" "+ fish[numFish];
}
}
Stacktrace：
java.lang.NullPointerException
at Fisher.likes(Fisher.java:34)
at Fisher.goFishingIn(Fisher.java:53)
at FishingTestProgram2.main(FishingTestProgram2.java:28)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
有人可以告诉我为什么p.catchAFish()在likes(Fish f1)中的类型不同？
因为我认为catchAFish方法将返回一个Fish对象，它与likes方法中的输入类型相同。
public class Fish {
private String species;
private int size;
public String getSpecies(){
return species;
}
public int getSize(){
return size;
}
public Fish(int s, String ss){
size=s;
species=ss;
}
public String toString(){
return "A " + size+"cm "+ species;
}
}
测试类：
public class FishingTestProgram2 {
public static void main(String [] args) {
// Create a big pond with 15 fish
Pond bigPond = new Pond(15);
bigPond.add(new Fish(4, "Sunfish"));
bigPond.add(new Fish(25, "Pike"));
bigPond.add(new Fish(20, "Bass"));
bigPond.add(new Fish(30, "Perch"));
bigPond.add(new Fish(14, "Sunfish"));
bigPond.add(new Fish(15, "Pike"));
bigPond.add(new Fish(9, "Pike"));
bigPond.add(new Fish(12, "Bass"));
bigPond.add(new Fish(5, "Sunfish"));
bigPond.add(new Fish(12, "Sunfish"));
bigPond.add(new Fish(10, "Bass"));
bigPond.add(new Fish(2, "Bass"));
bigPond.add(new Fish(16, "Perch"));
bigPond.add(new Fish(30, "Sunfish"));
bigPond.add(new Fish(7, "Perch"));
bigPond.listFish();
// Create two people to fish in the pond
Fisher fred = new Fisher("Fred", 15);
Fisher suzy = new Fisher("Suzy", 10);
System.out.println("First Fred catches 20 fish in the big pond ...");
for (int i=0; i<20; i++)
fred.goFishingIn(bigPond);
fred.listFish();
System.out.println("Suzy now catches 20 fish in the big pond ...");
for (int i=0; i<20; i++)
suzy.goFishingIn(bigPond);
suzy.listFish();
System.out.println("Here is what is left of the pond ...");
bigPond.listFish();
// Now simulate Suzy giving her fish to Fred
suzy.giveAwayFish(fred, bigPond);
fred.listFish();
suzy.listFish();
bigPond.listFish();
}
}
字符串比较行是正确的我在我试图解决问题时更改了它。 调试说明了这一点。指鱼和p。 means（int，string）我需要在同一类型中初始化这两个东西，但我不知道如何更改它。
答案 0 :(得分：2)
catchAFish()返回null，因此当您传递likes()时，if(f1.getSpecies()=="Sunfish")在调用if(likes(p.catchAFish())){ // the problem happens here时会在相似内容中获得空引用。
我不知道如何调用goFishingIn()，你不提供该代码。但是你最有可能用一个悲伤的空池来称呼它。
您需要在使用catchAFish的返回值之前检查null，或者确保catchAFish永远不会返回null。现在它会。
好的 - 我看到了你的主要功能。问题是你出去钓鱼了。
public Fish catchAFish(){
int num= (int)(Math.random() * (numFish-1));
if(fish[num]!=null && numFish>0){
fff= fish[num];
fish[num]= fish[numFish-1];
fish[numFish-1]=null;
numFish--;
return fff;
}
else
return null;
}
你将你的数组初始化为20，但是每次钓鱼你都会删除整个Fish类，所以当你打电话给goFishingIn时，你要从池塘中的鱼群中移除一个Fish对象。
我想你想要去掉一条鱼，一旦鱼类全部被捕获[num] == 0，那就去除鱼。
public Fish catchAFish(){
int num= (int)(Math.random() * (numFish-1));
if(fish[num]!=null && numFish>0){
fff= fish[num];
fish[num]--;
if (fish[num] == 0)
{
// No more of this fish type, remove it from the pond.
fish[num]= fish[numFish-1];
fish[numFish-1]=null;
numFish--;
}
return fff;
}
else
return null;
}