[TOC]
方式一：+ 最常見的方式
String aa = "今天";
String bb = "明天";
System.out.println(aa+bb);
方式二：StringBuilder.append()和StringBuffer.append()
先有StringBuffer后有StringBuilder，兩者就像是孿生雙胞胎，該有的都有间护，只不過大哥StringBuffer，大部分方法都經(jīng)過synchronized修飾李皇，所以StringBuffer是線程安全的扰才，但是它效率就相對StringBuilder較低
String aa = "今天";
String bb = "明天";
StringBuilder sber = new StringBuilder();
StringBuffer sbf = new StringBuffer();
sber.append(aa).append(bb);
System.out.println(sber.toString());
sbf.append(aa).append(bb);
System.out.println(sbf.toString());
方式三：String類下的cocat()方法
如果拼接的字符串是null，concat會拋出NullPointerException汽馋。如果拼接的字符串是一個空字符串（“”）明场，那么concat的效率要更高汽摹。如果拼接的字符串非常多，concat的效率就會下降苦锨，因為創(chuàng)建的字符串對象越多逼泣，開銷越大。
String aa = "今天";
String bb = "明天";
String concat = aa.concat(bb);
System.out.println(concat);
方式四：String類下的join()方法
JDK1.8提供了一種新的字符串拼接姿勢：String類增加了一個靜態(tài)方法join舟舒，第一個參數(shù)為字符串連接符
String aa = "今天";
String bb = "明天";
String join = String.join("-", aa, bb);
System.out.println(join);
方式五：StringJoiner
StringJoiner是JDK1.8拉庶，java.util包中的一個類，用于構(gòu)造一個由分隔符重新連接的字符序列
String aa = "今天";
String bb = "明天";
StringJoiner sj = new StringJoiner(":", "[", "]");
sj.add(aa).add(bb);
System.out.println(sj.toString());
StringJoiner源碼
package java.util;
public final class StringJoiner {
private final String prefix;//前綴
private final String delimiter;//間隔符
private final String suffix;//后綴
private StringBuilder value;//值
private String emptyValue;//空值
public StringJoiner(CharSequence delimiter) {
this(delimiter, "", "");//默認前綴和后綴為"",重載調(diào)用
}
public StringJoiner(CharSequence delimiter,
CharSequence prefix,
CharSequence suffix) {
//間隔符秃励，前綴和后綴判斷是否為null氏仗，null將拋出異常
Objects.requireNonNull(prefix, "The prefix must not be null");
Objects.requireNonNull(delimiter, "The delimiter must not be null");
Objects.requireNonNull(suffix, "The suffix must not be null");
// 成員變量賦值
this.prefix = prefix.toString();
this.delimiter = delimiter.toString();
this.suffix = suffix.toString();
this.emptyValue = this.prefix + this.suffix;//空值被設(shè)置為只有前后綴
}
//設(shè)置空值，檢查是否為null
public StringJoiner setEmptyValue(CharSequence emptyValue) {
this.emptyValue = Objects.requireNonNull(emptyValue,
"The empty value must not be null").toString();
return this;
}
@Override
public String toString() {
if (value == null) {
return emptyValue;//沒有值將返回空值或者后續(xù)設(shè)置的空值
} else {
if (suffix.equals("")) {
return value.toString();//后綴為""直接返回字符串，不用添加
} else {
//后綴不為""皆尔，添加后綴呐舔，然后直接返回字符串，修改長度
int initialLength = value.length();
String result = value.append(suffix).toString();
// reset value to pre-append initialLength
value.setLength(initialLength);
return result;
}
}
}
初始化慷蠕，先添加前綴珊拼，有了之后每次先添加間隔符，StringBuilder后續(xù)append字符串
public StringJoiner add(CharSequence newElement) {
prepareBuilder().append(newElement);
return this;
}
//合并StringJoiner流炕，注意后面StringJoiner 的前綴就不要了澎现，后面的appen進來
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;
}
//初始化，先添加前綴每辟，有了之后每次先添加間隔符
private StringBuilder prepareBuilder() {
if (value != null) {
value.append(delimiter);
} else {
value = new StringBuilder().append(prefix);
}
return value;
}
public int length() {
// Remember that we never actually append the suffix unless we return
// the full (present) value or some sub-string or length of it, so that
// we can add on more if we need to.
//不忘添加后綴的長度
return (value != null ? value.length() + suffix.length() :
emptyValue.length());
}
}
方式六：StringUtils.join()
實戰(zhàn)項目中剑辫，我們處理字符串的時候，經(jīng)常會用到這個類.org.apache.commons.lang3.StringUtils包下渠欺，該方法更善于拼接數(shù)組中的字符串妹蔽，并且不用擔心 NullPointerException。
String aa = "今天";
String bb = "明天";
String ids[] = {"1","2","3"};
System.out.println(StringUtils.join(aa,bb,"-","124"));
String join1 = StringUtils.join(ids,",");
System.out.println(join1);
