Java record

Java

记录在开发过程中遇到的java常见小问题、细节问题。

获得二维数组长度¶

char[][] board
int n = board.length;
int m = n > 0 ? board[0].length : 0;

初始化二维数组¶

https://stackoverflow.com/questions/13832880/initialize-2d-array

private char[][] table = {{'1', '2', '3'}, {'4', '5', '6'}, {'7', '8', '9'}};

排序二维数组¶

按照第1个元素排序：

Arrays.sort(myArr, (a, b) -> Double.compare(a[0], b[0]));
Arrays.sort(queries, Comparator.comparing(a -> a[0]));
Arrays.sort(queries, (a, b) -> a[0] - b[0]);

打印数组¶

https://stackoverflow.com/questions/409784/whats-the-simplest-way-to-print-a-java-array

System.out.println(Arrays.toString(array));
Nested Array:
System.out.println(Arrays.deepToString(deepArray));

java中int转成String位数不足前面补零¶

String.format("%06",12);//其中0表示补零而不是补空格，6表示至少6位

Java最小值和最大值¶

Integer.MIN_VALUE和Integer.MAX_VALUE

将List转化为数组¶

使用

List<String> list = new ArrayList<String>();
String[] a = list.toArray(new String[0]);

而不是

List<String> list = new ArrayList<String>();
...
String[] a = (String[]) list.toArray(list);

但是一下做法是错误的

List<Integer> list = new ArrayList<Integer>();
...
int[] a = list.toArray(new int[0]);

原因就在与int不能作为范型类型参数(use int as a type argument for generics)。所以只能利用Java8的新特性了：

int[] array = list.stream().mapToInt(i->i).toArray();

将数组转化为List¶

https://stackoverflow.com/questions/1073919/how-to-convert-int-into-listinteger-in-java

There is no shortcut for converting from int[] to List<Integer> as Arrays.asList does not deal with boxing and will just create a List<int[]> which is not what you want.

int[] ints = {1, 2, 3};
List<Integer> intList = new ArrayList<Integer>();
for (int i : ints) intList.add(i);

List<Integer> list = Arrays.stream(ints).boxed().collect(Collectors.toList());

Java数组拷贝¶

clone¶

clone方法是从Object类继承过来的，基本数据类型（String ，boolean，char，byte，short，float ，double，long）都可以直接使用clone方法进行克隆，注意String类型是因为其值不可变所以才可以使用。

int[] a1 = {1, 3};
int[] a2 = a1.clone();

System.arraycopy¶

public static native void arraycopy(Object src, int srcPos, 
        Object dest, int desPos, int length)

由于是native方法，所以效率非常高，在频繁拷贝数组的时候，建议使用。

反射¶

https://stackoverflow.com/questions/1196192/how-to-read-the-value-of-a-private-field-from-a-different-class-in-java

Field f = obj.getClass().getDeclaredField("stuffIWant"); //NoSuchFieldException
f.setAccessible(true);
Hashtable iWantThis = (Hashtable) f.get(obj); //IllegalAccessException

https://stackoverflow.com/questions/34571/how-do-i-test-a-private-function-or-a-class-that-has-private-methods-fields-or

Field field = TargetClass.getDeclaredField(fieldName);
field.setAccessible(true);
field.set(object, value);

native¶

在Class Object中一个wait()方法定义为：

public final native void wait(long timeout) throws InterruptedException;

这里的native修饰词说明其修饰的方法的实现，是用其他语言(C/C++)实现的，该方法通过JNI调用本地代码。

JNI(Java Native Interface, Java本地接口)使Java虚拟机中的Java程序可以调用本地代码。

JNI