Guave是一个强大的的工具集合，今天给大家介绍一下，常用的拼接字符串的方法，当然JDK 也有方便的拼接字符串的方式，本文主要介绍guava的，可以对比使用

基本的拼接的话可以如下操作

@Test
    public void test(){
        final List list1 = Arrays.asList("a","b","c");
        final String join1 = Joiner.on("#").join(list1);
        assertThat(join1,equalTo("a#b#c"));
      }

如果list 中含有null值，直接拼接会报错

可以使用如下的方式，跳过null值

 @Test
    public void test(){
    final List list2 = Arrays.asList("a","b","c",null);
        //skipNulls 跳过空值
        final String join2 = Joiner.on("#").skipNulls().join(list2);
        assertThat(join2,equalTo("a#b#c"));
    }

或者是给null值设置一个默认值

@Test
public void testUseForNull(){
    final List list2 = Arrays.asList("a","b","c",null);
    final String aDefault = Joiner.on("#").useForNull("DEFAULT").join(list2);
    assertThat(aDefault,equalTo("a#b#c#DEFAULT"));
}

拼接Map 键值对

@org.junit.Test
public void testWithKeyValueSeparator(){
    Map map = new HashMap<>(16);
    map.put("k1","v1");
    map.put("k2","v2");
    final String join = Joiner.on("#").withKeyValueSeparator("-").join(map);
    System.out.println(join);
    assertThat(join,equalTo("k1-v1#k2-v2"));
    }

JDK 拼接字符串，也可以实现 去掉null值或者是给予默认值

@Test
public void testJDKContactStringWithNull() {
    final List list1 = Arrays.asList("a", "b", "c", null, "d");
    //会把null 转为字符串 "null"，然后再拼接
    /**
     * 查看源码  Collectors::joining() ---> StringJoiner::add()
     * prepareBuilder().append()--->AbstractStringBuilder::appendNull()
     * appendNull 里面做的事情就是 插入了一个字符串  null，准确说是插入一个 charSequence null
     */
    final String joinStr = list1.stream().collect(Collectors.joining("#"));
    System.out.println(joinStr);
    assertThat(joinStr.equals("a#b#c#null#d"), equalTo(true));
    //如果想跳过null，可以使用filter
    final String joinStrNoNull = list1.stream()
            .filter(str -> StringUtils.isNoneBlank(str))
            .collect(Collectors.joining("#"));
    assertThat(joinStrNoNull.equals("a#b#c#d"), equalTo(true));
}


@Test
public void testJDKContactStringWithDefaultForNull() {
    final List list1 = Arrays.asList("a", "b", "c", null, "d");

    final String strWithDefault = list1.stream()
            .map(str -> {
                if (StringUtils.isEmpty(str)) {
                    str = "DEFAULT";
                }
                return str;
            })
            .collect(Collectors.joining("#"));
    assertThat(strWithDefault.equals("a#b#c#DEFAULT#d"), equalTo(true));
}