[英]Is there a static method for checking if any method in an invocation chain returns null?
[英]Finding the null in a method invocation chain
我正在寻找一种快速的方法来确定链的哪个部分为空。
举例说明这一点:
public class Chain {
private Chain chain;
public Chain returnChain() {
return chain;
}
public void addChain(Chain chain) {
this.chain=chain;
}
public String toString() {
return "Hello!";
}
public static void main(String[] args) {
Chain c1 = new Chain();
c1.addChain(new Chain());
System.out.println(c1.returnChain().returnChain().returnChain().returnChain());
}
}
这显然会抛出NullPointerException
。 (我知道如何更改代码以明确链接的哪一部分抛出NullPointerException
,但我想用现有代码来解决这个问题。)
在典型NPE的堆栈跟踪中,您将获得它所发生的行号。 假设System.out.println(c1.returnChain().returnChain().returnChain().returnChain());
在第144行(只是随机选取)你的NPE堆栈跟踪应该类似于:
java.lang.NullPointerException
at your.package.Chain(Chain.java:144)
因此,如果您将链调用放在多行上,它应该显示异常的位置。
我写了这个小例子:
package bla;
public class Something {
public static int count = 0;
public Something get() {
if(count == 2) {
return null;
}
++count;
return new Something();
}
public static void main(String[] args){
Something something = new Something();
Something test = something.get()
.get()
.get()
.get() // Should throw NPE
.get()
.get();
}
}
它给了我NPE说:在bla.Something.main(Something.java:18) - 确切地说NPE在链中发生的地方。
屏幕截图......
对于您拥有的特定代码,请尝试将以下方法添加到您的Chain
类:
public static Chain checkChainSequence(Chain first, int count) {
Chain thisChain = first;
StringBuilder out = new StringBuilder("firstChain");
for (int i = 0; i < count; i++) {
Chain nextChain = thisChain.returnChain();
out.append(".returnChain()");
if (nextChain == null) {
out.append(" returned null");
System.out.println(out);
return null;
}
thisChain = nextChain;
}
return thisChain;
}
您可以按如下方式使用它:
Chain c1 = new Chain();
c1.addChain(new Chain());
// To check c1.returnChain().returnChain().returnChain():
Chain.checkChainSequence(c1, 3);
这将打印:
firstChain.returnChain().returnChain() returned null
也许我没有得到它,但只是returnChain() == null
?
您可以添加Chain
类变量,如position
用于确定链中元素的位置:
public class Chain {
private Chain chain;
private int position;
public Chain returnChain() {
if (chain == null) {
System.out.println(position + " chain is null");
} else {
chain.position = position + 1;
}
return chain;
}
public void addChain(Chain chain) {
this.chain=chain;
}
public String toString() {
return "Hello!";
}
public static void main(String[] args) {
Chain c1 = new Chain();
c1.addChain(new Chain());
c1.position = 0;
System.out.println(c1.returnChain().returnChain().returnChain().returnChain());
}
}
当然它不是线程安全的,看起来很奇怪。 但是应该写出null的位置。 更简单的版本:
public int findPositionOfNullChain(Chain chain) {
int position = 0;
while (chain != null) {
chain = chain.returnChain();
position++;
}
return position;
}
在不修改Chain类的情况下,您可以执行此操作以找出哪个部分为null:
Chain c1 = new Chain();
c1.addChain(new Chain());
Chain preNullChain = c1;
Chain returnedChained = null;
while ((returnedChained = preNullChain.returnChain()) != null) {
preNullChain = returnedChained;
}
//here, preNullChain is the last Chain that is not null
或者您可以捕获空指针异常以打印null
Chain returnedChain = null;
Chain c1 = new Chain();
c1.addChain(new Chain());
try {
returnedChain = c1.returnChain().returnChain().returnChain().returnChain();
} catch (NullPointerException e) {
}
System.out.println(returnedChain); //now here will print null as what you asked for
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