List、Array、String类型互转-CFANZ编程社区

之前笔试的时候，输出结果个数不确定，果断选择了用List<Integer> list = new ArrayList<>();存储，最后要求返回int[]，直接调用了list.toArray(),报错了，该函数返回 Object[],果断加上参数list.toArray(new Integer[list.size()]),结果返回Integer[],最后时间来不及了，也没做出来，下来查资料写下这篇文章铭记。

数组类型转换

测试数据

//【字符串】切成【数组】
String[] arr = "1,2,3,4,5,6".split(",");

字符串数组 to （Integer、Long、Double）列表

传统方式

1.【String数组】转成【Long数组】这里使用到了 Apache Commons BeanUtils 不过多介绍

//【String数组】转成【Long数组】
Long[] LongIds= (Long[]) ConvertUtils.convert(arr, Long.class);

2.【Long数组】转【ArrayList】

// Arrays.asList 返回固定大小的列表。
List<Long> list = Arrays.asList(LongIds);
// new ArrayList 创建动态的 ArrayList 
ArrayList<Long> ids = new ArrayList<Long>(list);

Java8 Stream方式

List<Integer> intList = Arrays.stream(arr).map(Integer::valueOf).collect(toList());

System.out.println(intList);

List<Long> longList = Arrays.stream(arr).map(Long::valueOf).collect(toList());

System.out.println(longList);

List<Double> doubleList = Arrays.stream(arr).map(Double::valueOf).collect(toList());

System.out.println(doubleList);

数组 to 数组

字符串数组 to （int、long、double ）数组

int[] ints = Arrays.stream(arr).mapToInt(Integer::valueOf).toArray();

long[] longs = Arrays.stream(arr).mapToLong(Long::valueOf).toArray();

double[] doubles = Arrays.stream(arr).mapToDouble(Double::valueOf).toArray();

int[]、long[]、double[] 数组互相转换

// int[] 2 long[]
long[] longs = Arrays.stream(ints).mapToLong(Long::valueOf).toArray();

// int[] 2 double[]
double[] doubles = Arrays.stream(ints).mapToDouble(Double::valueOf).toArray();

// long[] 2 int[]
int[] ints1 = Arrays.stream(longs).mapToInt(i -> (int)i).toArray();

// long[] 2 double[]
double[] doubles1 = Arrays.stream(longs).mapToDouble(Double::valueOf).toArray();

// double[] 2 int[]
int[] ints2 = Arrays.stream(doubles).mapToInt(i -> (int)i).toArray();

// double[] 2 long[]
long[] longs2 = Arrays.stream(doubles).mapToLong(i -> (long)i).toArray();

int[]、long[]、double[] 数组再转回 String[]

String[] strArr1 = Arrays.stream(ints).mapToObj(String::valueOf).toArray(String[]::new);

String[] strArr2 = Arrays.stream(longs).mapToObj(String::valueOf).toArray(String[]::new);

String[] strArr3 = Arrays.stream(doubles).mapToObj(String::valueOf).toArray(String[]::new);

基础类型数组 to 列表

Java8 以前用乖乖 for 循环

int[] intArr = {1, 2, 3, 4, 5, 6};
List ints = new ArrayList<>();
for (int i : intArr) {
	ints.add(i);
}
log.info("{}", ints); // [1, 2, 3, 4, 5, 6]

Java8 以后用 stream + boxed

int数组 to 列表

int[] intArr = {1, 2, 3, 4, 5, 6};

List<Integer> intList = Arrays.stream(intArr).boxed().collect(Collectors.toList());

System.out.println(intList); // [1, 2, 3, 4, 5, 6]

long数组 to 列表

long[] longArr = {1L, 2L, 3L, 4L, 5L, 6L};

List<Long> longList = Arrays.stream(longArr).boxed().collect(Collectors.toList());

System.out.println(longList); // [1, 2, 3, 4, 5, 6]

double数组 to 列表

double[] doubleArr = {1d, 2d, 3d, 4d, 5d, 6d};

List<Double> doubleList = Arrays.stream(doubleArr).boxed().collect(Collectors.toList());

System.out.println(doubleList); // [1.0, 2.0, 3.0, 4.0, 5.0, 6.0]

String数组 to 列表

String[] arr = "1,2,3,4,5,6".split(",");
List<String> strList  = Arrays.stream(arr).collect(Collectors.toList());
System.out.println(strList ); // [1, 2, 3, 4, 5, 6]

String[] arr = "1,2,3,4,5,6".split(",");
// list 不可编辑。
List<String> list = Arrays.asList(arr);
System.out.println(list); // [1, 2, 3, 4, 5, 6]
// 下面会报错，因为 asList 返回的是 Arrays 内部类 ArrayList 它没实现 add
// ArrayList<String>  list = Arrays.asList(arr); 

// arrayList  不可编辑
ArrayList<String> arrayList = new ArrayList<String>(list);
System.out.println(arrayList); // [1, 2, 3, 4, 5, 6]

// Collections.addAll 一切正常
List<String> ints = new ArrayList<>();
Collections.addAll(ints, arr);
log.info("{}", ints); // [1, 2, 3, 4, 5, 6]

列表 to 数组

List to Object[]

Object[] objects = new ArrayList<Integer>(list).toArray();
System.out.println(Arrays.toString(objects)); // [1, 2, 3, 4, 5]

List to Integer[]

Integer[] integers = new ArrayList<Integer>(list).toArray(new Integer[0]);
System.out.println(Arrays.toString(integers)); // [1, 2, 3, 4, 5]

List to int[]

List<Integer> list = new ArrayList<>();
int[] ints = list.stream().mapToInt(Integer::intValue).toArray();

数组转 String

Arrays.toString

注意两边有中括号[]

//======================= Array ==============================

String[] arr = {"你好", "哈喽", "啦啦啦"};

String str1 = Arrays.toString(arr);

System.out.println(str1); // [你好, 哈喽, 啦啦啦]

String.join

String[] arr = {"你好", "哈喽", "啦啦啦"};

//======================= Array ==============================
String str3 = String.join(", ", arr);
System.out.println(str3); // 你好, 哈喽, 啦啦啦

//======================= List<T> ==============================
List<String> list = Arrays.asList(arr);
String string2 = String.join(" - ", list);
System.out.println(string2); // 你好 - 哈喽 - 啦啦啦

Collectors.joining

//======================= List<T> ==============================
String[] arr = {"你好", "哈喽", "啦啦啦"};
List<String> list = Arrays.asList(arr);
String string1 = list.stream().collect(Collectors.joining("; "));
System.out.println(string1); // 你好; 哈喽; 啦啦啦