背景
在平时的业务开发中,我们可能会遇到字符串列表根据分隔符进行拼接的需求。比如:
输入:
数组:[“a”,“b”,“c”]分隔符:","输出:
“a,b,c”
处理
通常我们可以使用StringBuilder根据下标位置决定是否需要添加分隔符以达到目的,比如:
public static void main(String[] args) {
StringBuilder sb = new StringBuilder();
String[] strings = new String[]{"a", "b", "c"};
String delimiter = ",";
for (int i = 0; i < strings.length; i++) {
if (i != 0) {
sb.append(delimiter);
}
sb.append(strings[i]);
}
System.out.println(sb.toString());
}
或者:
public static void main(String[] args) {
StringBuilder sb = new StringBuilder();
String[] strings = new String[]{"a", "b", "c"};
String delimiter = ",";
int tmp = strings.length - 1;
for (int i = 0; i < strings.length; i++) {
sb.append(strings[i]);
if (i != tmp) {
sb.append(delimiter);
}
}
System.out.println(sb.toString());
}
但是每次遇到这种情况都去这样拼接很麻烦,而且如果使用迭代器,没有下标就不能使用这种方法。那么就每次append分隔符,最后再截断:
public static void main(String[] args) {
StringBuilder sb = new StringBuilder();
String[] strings = new String[]{"a", "b", "c"};
String delimiter = ",";
for (String str:strings) {
sb.append(str).append(delimiter);
}
System.out.println(sb.substring(0,sb.length()-1));
}
所以通常情况下,我们都会去扩展一个StringBuilder以提供分隔符组装的功能,博主平时是这样弄的:
public class MyStringBuilder {
private StringBuilder stringBuilder;
private String delimiter;
public MyStringBuilder(String delimiter) {
this.delimiter = delimiter;
}
public MyStringBuilder append(String item) {
getStringBuilder().append(item);
return this;
}
private StringBuilder getStringBuilder() {
if (stringBuilder == null) {
//相当于index==0,
stringBuilder = new StringBuilder();
} else {
stringBuilder.append(delimiter);
}
return stringBuilder;
}
@Override
public String toString() {
return stringBuilder == null ? "" : stringBuilder.toString();
}
}
在初始化的时候指定分隔符,然后每次直接append就行了,由内部决定在何时添加分隔符。
在JDK1.8之后,提供了一个StringJoiner类,提供了类似的功能,可以在初始化的时候指定分隔符和前缀后缀:
StringJoiner sj = new StringJoiner(",","(",")");
sj.add("a").add("b").add("c");
System.out.println(sj.toString());
//输出:(a,b,c)
博主大致看了一下StringJoiner的实现逻辑,很简单,和咱们扩展StringBuilder基本是一样的逻辑,另外加入了前缀后缀,然后多了一个merge的功能:
public StringJoiner merge(StringJoiner other) {
Objects.requireNonNull(other);
if (other.value != null) {
final int length = other.value.length();
// lock the length so that we can seize the data to be appended
// before initiate copying to avoid interference, especially when
// merge 'this'
StringBuilder builder = prepareBuilder();
builder.append(other.value, other.prefix.length(), length);
}
return this;
}
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