將數據從System.out.print方法存儲到字符串（Java）中？

Question

我正在制作密碼生成器和管理員。 我通過在for循環中一次生成一個字符來創建密碼。 在每個for循環迭代結束時，它將打印出所選的隨機字符。 我一直在嘗試將密碼char逐個char存儲到與for循環中i的值相關的數組的索引中。 那不僅僅是打印，而我卻無能為力。 因此，我有兩個問題，我的主要問題是：是否可以捕獲我打印的字符，然后將它們存儲在字符串中？ 或者，如下面的代碼所示，是否可以避免運行時（通過數組）獲得的空指針異常？ 當我嘗試在索引i處為我的passwordString分配字符串時，會發生空指針異常。 我的代碼可能會更好地解釋它。 謝謝你的幫助 ：）

package pass.gen;


public class PassGen {


    public static void main(String[] args) {
        PassGen passGen = new PassGen();
        passGen.generate();
    }

    String lAlpha = "abcdefghijklmnopqrstuvwxyz"; //used to generate lowercase pass chars
    String uAlpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; //used to generate uppercase pass chars
    int minNum = 0; //the mininum number that a integer value can be in the password
    int maxNum = 9; //the max number "" ...........................................""
    int randomNumber; 
    int minString = 0;
    int maxString = 25;
    int randomLNum;
    int randomUNum;
    int low1 = 1;
    int high3 = 3;
    int ran3;
    char randomLChar;
    char randomUChar;
    char randomNumChar;
    String randomLString;
    String randomUString;
    String randomNumString;
    String passString []; //should this be an array of chars? 
    boolean case1; //if the case is true (case1 is for random numbers) then a random number is added to the passString
    boolean case2; //if the case is true (case2 is for random lowercase letters) then a random lowercase letter is added to the passString
    boolean case3; //if the case is true (case3 is for random uppercase letters) then a random uppercase letter is added to the passString

    void generate(){
        for(int i = 0; i < 4; i++){
            ran3 = low1 + (int)(Math.random() * ((high3 - low1) + 1));
            switch(ran3){
            case 1:genNumber(0,9);
            break;
            case 2:genLAlpha();
            break;
            case 3:genUAlpha();
            break;
            default:System.out.println("Unable to Generate a Password.");
            }
            if(case1 == true){
                passString[i] = randomNumString;

            }if(case2 == true){
                passString[i] = randomLString;

            }if(case3 == true){
                passString[i] = randomUString;
            }
        }
        System.out.println(passString);
    }

    void genNumber(int min, int max){
        randomNumber = min + (int)(Math.random() * ((max - min) + 1));
        randomNumChar = Character.forDigit(randomNumber,5);
        case1 = true;
    }

    void genLAlpha(){
        randomLNum = minString + (int)(Math.random() * ((maxString - minString) + 1));
        randomLChar = lAlpha.charAt(randomLNum);
        randomLString = Character.toString(randomLChar);
        case2 = true;
    }

    void genUAlpha(){
        randomUNum = minString + (int)(Math.random() * ((maxString - minString) + 1));
        randomUChar = uAlpha.charAt(randomUNum);
        randomUString = Character.toString(randomUChar);
        case3 = true;
    }


}

Answer 1

• passString未初始化，因此導致NullPointerException的原因
• genNumber不設置的值randomNumString ，因此它永遠是空

您的“生成”方法應返回它們生成的值。 這將消除對成員變量的依賴，從而減少可能出現問題的區域。 這也意味着您可以添加新的生成方法，而無需更改很多代碼。

即使您選擇不執行此操作，您也應該有一個變量，其中包含每個計算的結果...

試試這個...

public class PassGen {

    public static void main(String[] args) {
        PassGen passGen = new PassGen();

        String generate = passGen.generate();
        System.out.println(generate);

    }
    String lAlpha = "abcdefghijklmnopqrstuvwxyz"; //used to generate lowercase pass chars
    String uAlpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; //used to generate uppercase pass chars
    int minNum = 0; //the mininum number that a integer value can be in the password
    int maxNum = 9; //the max number "" ...........................................""
    int minString = 0;
    int maxString = 25;
    int low1 = 1;
    int high3 = 3;

    public String generate() {
        StringBuilder sb = new StringBuilder(4);

        for (int i = 0; i < 4; i++) {
            sb.append(generateCharacter());
        }
        return sb.toString();
    }

    protected char generateCharacter() {
        char result = '-';
        int ran3 = low1 + (int) (Math.random() * ((high3 - low1)));
        switch (ran3) {
            case 0:
                result = genNumber(0, 9);
                break;
            case 1:
                result = genLAlpha();
                break;
            case 2:
                result = genUAlpha();
                break;
        }

        return result;

    }

    protected char genNumber(int min, int max) {
        int randomNumber = min + (int) (Math.random() * ((max - min) + 1));
        char randomNumChar = Character.forDigit(randomNumber, 5);
        return randomNumChar;
    }

    protected char genLAlpha() {
        int randomLNum = minString + (int) (Math.random() * ((maxString - minString) + 1));
        char randomLChar = lAlpha.charAt(randomLNum);
        return randomLChar;
    }

    protected char genUAlpha() {
        int randomUNum = minString + (int) (Math.random() * ((maxString - minString) + 1));
        char randomUChar = uAlpha.charAt(randomUNum);
        return randomUChar;
    }
}

另外，我強烈建議您花一些時間來學習如何使用IDE的調試器。 如果您不使用和IDE，強烈建議您開始使用。 我花了30秒添加一個斷點並找到您的NullPointerException