Java从字符串数组中随机生成字符串值

Question

我正在尝试从数组自动生成字符串值并将它们添加到另一个数组，但我有一个 nullPointer 异常。

protected static Random generator = new Random();

protected static String[] townNames = { "Paris", "Sofia", "Wiena", "Amsterdam" };
protected static String[] arr;

public static void main(String[] args) {

    for (int i = 0; i < towns.size(); i++) {
        arr[i] = townNames[generator.nextInt(townNames.length)];
    }

    for (String i : arr) {
        System.out.println(i);
    }

}

编辑

我已经编辑了代码，现在它可以按我的预期工作。

protected static Random generator = new Random();
protected static ArrayList<String> townNames = new ArrayList<>();

protected static ArrayList<String> generatedTowns = new ArrayList<String>();

public static void main(String[] args) {
    generatedArr(generatedTowns);
}

public static void generatedArr(ArrayList<String> a) {
    townNames.add("Paris");
    townNames.add("Sofia");
    townNames.add("Wiena");
    townNames.add("Amsterdam");
    townNames.add("Chikago");
    townNames.add("Vienna");
    townNames.add("Miami");
    townNames.add("Boston");
    townNames.add("New Orleans");
    for (int i = 0; i < 4; i++) {
        String value = townNames.get(generator.nextInt(townNames.size()));
        generatedTowns.add(value);
        townNames.remove(value);
    }

    for (String i : a) {
        System.out.println(i);
    }
}

Answer 1

• 您必须先创建数组才能使用它，并添加与 townNames 大小相同的随机元素。就像这样 arr=new String[townNames.length];

您可以尝试以下代码:

protected static Random generator = new Random();  
protected static String[] townNames = { "Paris", "Sofia", "Wiena", "Amsterdam" };
protected static String[] arr;

public static void main(String[] args) {
// We create an array with same size as a townNames 
arr=new String[townNames.length];

for (int i = 0; i < townNames.length; i++) {
arr[i] = townNames[generator.nextInt(townNames.length)];
}

for (String i : arr) {
System.out.println(i);
}
}