新人问题。Exception in thread "main" java.lang.NullPointerException

[mingcai1001]

最近开始学习java, 做作业遇到理解不了的问题，请教一下。
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public class Assign3 {

public static void main(String[] args) {
// TODO Auto-generated method stub
CodeBreaker a3 = new CodeBreaker();
a3.display();

} // end of method
} // end if class

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public class CodeBreaker {
private CodeLetter letter1;
private CodeLetter letter2;
private CodeLetter letter3;

public CodeBreaker(){
CodeLetter letter1 = new CodeLetter();
CodeLetter letter2 = new CodeLetter();
CodeLetter letter3 = new CodeLetter();
letter1.display();
letter2.display();
letter3.display();
System.out.println();
}

public void display(){
System.out.print("The code was ");
letter1.display();
letter2.display();
letter3.display();

}
}// end of class

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import java.util.Random;

public class CodeLetter {
private char letterValue;
Random rnd = new Random();

public CodeLetter(){
letterValue = (char) (rnd.nextInt(5)+'A');
}

public void display(){
System.out.print(letterValue);
}

}
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运行结果：
DCB
Exception in thread "main" java.lang.NullPointerException
at CodeBreaker.checkGuess(CodeBreaker.java:18)
at Assign3.main(Assign3.java:6)

我的问题是，为什么在public class CodeBreaker里，同样是调用public class CodeLetter的public void display()，在public CodeBreaker()可以打印字母（DCB），但是在public void display()却出错，不能打印了呢？

[伫望-向北]

public CodeBreaker(){ CodeLetter letter1 = new CodeLetter(); CodeLetter letter2 = new CodeLetter(); CodeLetter letter3 = new CodeLetter(); letter1.display(); letter2.display(); letter3.display(); System.out.println(); } 改成 public CodeBreaker(){ letter1 = new CodeLetter(); letter2 = new CodeLetter(); letter3 = new CodeLetter(); letter1.display(); letter2.display(); letter3.display(); System.out.println(); } 就可以了.

[mingcai1001]

多谢。没留意到这个错误。。。