guava Joiner和MapJoiner线程安全

{@code Joiner} 和 {@code MapJoiner}　的实例总是 immutable（详见java并发编程实战相关部分）

[ 1.状态创建后不能被修改． ２．所有的域都是final类型，并且它被正确创建（创建期间没有发生this引用的逸出） ]

创建通过静态函数on，内部调用构造函数，对于设置null类型的时候，函数内部会生成一个匿名类，从而重载设置null类型的相关函数．

所以他们都是线程安全的． 他们的构造函数都是私有的，必须调用他们的静态方法生成他们的对象（不用反射构造对象情况下）

public class Chapter2Code {

private final List<String> list = Lists.newArrayList("name", "doctor", "sex", "man", null);

@Rule
public ExpectedException expectedException = ExpectedException.none();

@Test
public void test_Joiner() {

    Joiner joiner = Joiner.on(",").skipNulls();

    assertThat(joiner.join(list), equalTo("name,doctor,sex,man"));

    joiner = Joiner.on(",").useForNull("null");
    assertThat(joiner.join(list), equalTo("name,doctor,sex,man,null"));

    joiner = Joiner.on("|");
    expectedException.expect(NullPointerException.class);// 如果不处理空指针，会抛出异常．
    joiner.join(list);

}

@Test
public void test_Joiner_wrong_use() {

    Joiner joiner = Joiner.on(",");
    joiner.skipNulls();// 这个方法会生成一个新的对象，对原对象的设置无影响
    expectedException.expect(NullPointerException.class);
    joiner.join(list);
}

@Test
public void test_Joiner_appendTo_StringBuilder() {
    Joiner joiner = Joiner.on("|").skipNulls();
    StringBuilder builder = new StringBuilder(256);

    joiner.appendTo(builder, list);

    assertThat(builder.toString(), equalTo("name|doctor|sex|man"));
    System.out.println(builder.toString());
}

@Test
public void test_Joiner_appendTo_File() throws IOException {
    File file = new File("a.txt");

    if (!file.exists()) {
        file.createNewFile();
    }

    try (FileWriter fileWriter = new FileWriter(file);) {
        Joiner.on("#").skipNulls().appendTo(fileWriter, list);

    } catch (Exception e) {
        throw new RuntimeException(e);
    }

    List<String> lines = Files.readAllLines(Paths.get("a.txt"));

    System.out.println(lines);
    assertThat(Joiner.on("").join(lines), equalTo(Joiner.on("#").skipNulls().join(list)));

}

@Test
public void test_MapJoiner() {
    Map<String, String> map = Maps.newLinkedHashMap();
    map.put("name", "doctor");
    map.put("sex", "man");

    String join = Joiner.on(",").withKeyValueSeparator("=").join(map);
    assertThat(join, equalTo("name=doctor,sex=man"));
}
}