关于java System.in的一个异常

关于java System.in的一个错误？
下面的程序的作用是：给出n个整数，求出这n个整数中前k个最大的整数，比如：5个整数｛5.9.1.3.2｝，前3个最大的整数是：｛5.9.3｝
然后我的程序出现了这样的错误（控制台输出！）：

this is temp constructor!
Input n:
Exception in thread "main" java.lang.NullPointerException
at datastruct.Temp.setN(Temp.java:40)
at datastruct.Temp.<init>(Temp.java:17)
at datastruct.Temp.main(Temp.java:88)

Java code

import java.util.Scanner;/* * 给定n个整数，求出其中k个最大的数 */public class Temp {    private int k;    private int n;    private int[] intInputArray;    private int[] intSomeMaxNumbersArray;    private Scanner sca;    public Temp() {        System.out.println("this is temp constructor!");        this.setN();        this.setK();        this.intInputArray = new int[this.getN()];        this.intSomeMaxNumbersArray = new int[this.getK()];        this.initIntInputArray();        sca = new Scanner(System.in);    }    public int getK() {        return k;    }    public void setK() {        System.out.println("Input k:");        k = sca.nextInt();    }    public int getN() {        return n;    }    public void setN() {        System.out.println("Input n:");        n = sca.nextInt();    }    private void initIntInputArray() {        System.out.println("Input IntInputArray:");        for (int i = 0; i < this.getN(); i++) {            this.intInputArray[i] = sca.nextInt();        }    }    public void operation() {        int[] minNumAndIndex = new int[2];        for (int i = 0; i < this.getN(); i++) {            if (i < this.getK()) {                this.intSomeMaxNumbersArray[i] = this.intInputArray[i];            } else {                minNumAndIndex = this.getMinNumIndex();                if (minNumAndIndex[0] < this.intInputArray[i]) {                    this.intSomeMaxNumbersArray[minNumAndIndex[1]] = this.intInputArray[i];                }            }        }    }    private int[] getMinNumIndex() {        int minIndex;        int min;        int[] minNumAndIndex = new int[2];        min = this.intSomeMaxNumbersArray[0];        minIndex = 0;        for (int i = 0; i < this.getK(); i++) {            if (min >= this.intSomeMaxNumbersArray[i]) {                min = this.intSomeMaxNumbersArray[i];                minIndex = i;            }        }        minNumAndIndex[0] = min;        minNumAndIndex[1] = minIndex;        return minNumAndIndex;    }    private void printResult() {        for (int i = 0; i < this.getK(); i++) {            System.out.println(this.intSomeMaxNumbersArray[i] + " ");        }    }    public static void main(String args[]) {        Temp temp = new Temp();        temp.operation();        temp.printResult();    }}

[解决办法]
sca = new Scanner(System.in);
这条语句要前移,放在这落后于setN()了。
把它紧跟着放在 System.out.println("this is temp constructor!");
下面。
[解决办法]
出错在：n = sca.nextInt();
这时 sca=null.
sca.netxInt() 出异常了。
[解决办法]
在Temp构造器中调用this.setN时，scan还依然为null。所以在setN方法内用sca.nextInt();会包空指针的错误。
[解决办法]

Java code

this.setN(); //setN用到sca        this.setK();        this.intInputArray = new int[this.getN()];        this.intSomeMaxNumbersArray = new int[this.getK()];        this.initIntInputArray();        sca = new Scanner(System.in); //而sca在这里才初始化，所以setN就抛出空指针异常了 

 

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