关于java System.in的一个错误？

[SelfMedicated]

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)

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();
	}
}

[SelfMedicated]

有任何好的建议都欢迎提醒，不过关键还是这错误是怎么产生滴~~

[安特矮油]

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。

[qybao]

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

[SelfMedicated]

我了个去啊，sorry，浪费各位老师的时间了，没有仔细观察就来这问问题了，以后一定仔细调试后在来问。

[nmyangym]

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。

[nmyangym]

出错在：n = sca.nextInt();
这时 sca=null.
sca.netxInt() 出异常了。