使用String和使用String之间有明显的区别吗?格式和字符串连接在Java?
我倾向于使用String。格式，但偶尔会滑倒和使用连接。我想知道哪个比哪个好。
在我看来，String。Format让你在“格式化”字符串时更强大;连接意味着您不必担心不小心输入了额外的%s或遗漏了一个。
字符串。格式也更短。
哪一个更容易读，取决于你的大脑如何工作。
当前回答
错误的测试重复多次 不应该使用{}%s。
public static void main(String[] args) throws Exception {
long start = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
String s = "Hi " + i + "; Hi to you " + i * 2;
}
long end = System.currentTimeMillis();
System.out.println("Concatenation = " + ((end - start)) + " millisecond");
start = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
String s = String.format("Hi %s; Hi to you %s", i, +i * 2);
}
end = System.currentTimeMillis();
System.out.println("Wrong use of the message format = " + ((end - start)) + " millisecond");
start = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
String s = String.format("Hi {0}; Hi to you {1}", i, +i * 2);
}
end = System.currentTimeMillis();
System.out.println("Good use of the message format = " + ((end - start)) + " millisecond");
}
Concatenation = 88 millisecond
Wrong use of the message format = 1075 millisecond
Good use of the message format = 376 millisecond
其他回答
我没有做任何具体的基准测试，但我认为连接可能更快。String.format()创建一个新的Formatter，该Formatter反过来创建一个新的StringBuilder(大小只有16个字符)。这是一个相当大的开销，特别是当您正在格式化一个较长的字符串并且StringBuilder必须不断调整大小时。
然而，串联不太有用，也更难阅读。一如既往，有必要对代码进行基准测试，看看哪个更好。在服务器应用程序中，当你的资源包、locale等加载到内存中，代码被jit后，这些差异可以忽略不计。
也许作为一个最佳实践，它将是一个好主意，创建自己的Formatter与适当大小的StringBuilder(可追加的)和地区，并使用它，如果你有很多格式化要做。
由于有关于性能的讨论，我想我应该添加一个包含StringBuilder的比较。事实上，它比concat更快，当然也比String更快。格式选项。
为了使这成为一种苹果对苹果的比较，我在循环中而不是在外部实例化了一个新的StringBuilder(这实际上比只实例化一个更快，这很可能是由于在一个构建器的末尾为循环追加重新分配空间的开销)。
String formatString = "Hi %s; Hi to you %s";
long start = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
String s = String.format(formatString, i, +i * 2);
}
long end = System.currentTimeMillis();
log.info("Format = " + ((end - start)) + " millisecond");
start = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
String s = "Hi " + i + "; Hi to you " + i * 2;
}
end = System.currentTimeMillis();
log.info("Concatenation = " + ((end - start)) + " millisecond");
start = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
StringBuilder bldString = new StringBuilder("Hi ");
bldString.append(i).append("; Hi to you ").append(i * 2);
}
end = System.currentTimeMillis();
log.info("String Builder = " + ((end - start)) + " millisecond");
2012-01-11 16:30:46,058 INFO [TestMain] - Format = 1416毫秒 2012-01-11 16:30:46,190 INFO [TestMain] -连接= 134毫秒 2012-01-11 16:30:46,313 INFO [TestMain] - String Builder = 117毫秒
这是一个以毫秒为单位的多个样本大小的测试。
public class Time {
public static String sysFile = "/sys/class/camera/rear/rear_flash";
public static String cmdString = "echo %s > " + sysFile;
public static void main(String[] args) {
int i = 1;
for(int run=1; run <= 12; run++){
for(int test =1; test <= 2 ; test++){
System.out.println(
String.format("\nTEST: %s, RUN: %s, Iterations: %s",run,test,i));
test(run, i);
}
System.out.println("\n____________________________");
i = i*3;
}
}
public static void test(int run, int iterations){
long start = System.nanoTime();
for( int i=0;i<iterations; i++){
String s = "echo " + i + " > "+ sysFile;
}
long t = System.nanoTime() - start;
String r = String.format(" %-13s =%10d %s", "Concatenation",t,"nanosecond");
System.out.println(r) ;
start = System.nanoTime();
for( int i=0;i<iterations; i++){
String s = String.format(cmdString, i);
}
t = System.nanoTime() - start;
r = String.format(" %-13s =%10d %s", "Format",t,"nanosecond");
System.out.println(r);
start = System.nanoTime();
for( int i=0;i<iterations; i++){
StringBuilder b = new StringBuilder("echo ");
b.append(i).append(" > ").append(sysFile);
String s = b.toString();
}
t = System.nanoTime() - start;
r = String.format(" %-13s =%10d %s", "StringBuilder",t,"nanosecond");
System.out.println(r);
}
}
TEST: 1, RUN: 1, Iterations: 1
Concatenation = 14911 nanosecond
Format = 45026 nanosecond
StringBuilder = 3509 nanosecond
TEST: 1, RUN: 2, Iterations: 1
Concatenation = 3509 nanosecond
Format = 38594 nanosecond
StringBuilder = 3509 nanosecond
____________________________
TEST: 2, RUN: 1, Iterations: 3
Concatenation = 8479 nanosecond
Format = 94438 nanosecond
StringBuilder = 5263 nanosecond
TEST: 2, RUN: 2, Iterations: 3
Concatenation = 4970 nanosecond
Format = 92976 nanosecond
StringBuilder = 5848 nanosecond
____________________________
TEST: 3, RUN: 1, Iterations: 9
Concatenation = 11403 nanosecond
Format = 287115 nanosecond
StringBuilder = 14326 nanosecond
TEST: 3, RUN: 2, Iterations: 9
Concatenation = 12280 nanosecond
Format = 209051 nanosecond
StringBuilder = 11818 nanosecond
____________________________
TEST: 5, RUN: 1, Iterations: 81
Concatenation = 54383 nanosecond
Format = 1503113 nanosecond
StringBuilder = 40056 nanosecond
TEST: 5, RUN: 2, Iterations: 81
Concatenation = 44149 nanosecond
Format = 1264241 nanosecond
StringBuilder = 34208 nanosecond
____________________________
TEST: 6, RUN: 1, Iterations: 243
Concatenation = 76018 nanosecond
Format = 3210891 nanosecond
StringBuilder = 76603 nanosecond
TEST: 6, RUN: 2, Iterations: 243
Concatenation = 91222 nanosecond
Format = 2716773 nanosecond
StringBuilder = 73972 nanosecond
____________________________
TEST: 8, RUN: 1, Iterations: 2187
Concatenation = 527450 nanosecond
Format = 10291108 nanosecond
StringBuilder = 885027 nanosecond
TEST: 8, RUN: 2, Iterations: 2187
Concatenation = 526865 nanosecond
Format = 6294307 nanosecond
StringBuilder = 591773 nanosecond
____________________________
TEST: 10, RUN: 1, Iterations: 19683
Concatenation = 4592961 nanosecond
Format = 60114307 nanosecond
StringBuilder = 2129387 nanosecond
TEST: 10, RUN: 2, Iterations: 19683
Concatenation = 1850166 nanosecond
Format = 35940524 nanosecond
StringBuilder = 1885544 nanosecond
____________________________
TEST: 12, RUN: 1, Iterations: 177147
Concatenation = 26847286 nanosecond
Format = 126332877 nanosecond
StringBuilder = 17578914 nanosecond
TEST: 12, RUN: 2, Iterations: 177147
Concatenation = 24405056 nanosecond
Format = 129707207 nanosecond
StringBuilder = 12253840 nanosecond
我想我们可以用MessageFormat。格式，因为它应该在可读性和性能方面都很好。
我使用了与Icaro在上面的回答中使用的相同的程序，并通过添加使用MessageFormat来解释性能数字的代码来增强它。
public static void main(String[] args) {
long start = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
String s = "Hi " + i + "; Hi to you " + i * 2;
}
long end = System.currentTimeMillis();
System.out.println("Concatenation = " + ((end - start)) + " millisecond");
start = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
String s = String.format("Hi %s; Hi to you %s", i, +i * 2);
}
end = System.currentTimeMillis();
System.out.println("Format = " + ((end - start)) + " millisecond");
start = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
String s = MessageFormat.format("Hi %s; Hi to you %s", i, +i * 2);
}
end = System.currentTimeMillis();
System.out.println("MessageFormat = " + ((end - start)) + " millisecond");
}
串联= 69毫秒 格式= 1435毫秒 MessageFormat = 200毫秒
更新:
根据SonarLint报告，printf风格的格式字符串应该正确使用(squid:S3457)
Because printf-style format strings are interpreted at runtime, rather than validated by the compiler, they can contain errors that result in the wrong strings being created. This rule statically validates the correlation of printf-style format strings to their arguments when calling the format(...) methods of java.util.Formatter, java.lang.String, java.io.PrintStream, MessageFormat, and java.io.PrintWriter classes and the printf(...) methods of java.io.PrintStream or java.io.PrintWriter classes.
我用括号替换了printf样式，得到了一些有趣的结果，如下所示。
串联= 69毫秒格式= 1107毫秒 格式:括号= 416毫秒MessageFormat = 215 毫秒MessageFormat:括号= 2517毫秒
我的结论是: 如上所述，使用String。带尖括号的格式应该是一个很好的选择，以获得良好的可读性和性能。
下面是与上面相同的测试，只是修改了对StringBuilder调用toString()方法。下面的结果表明，StringBuilder方法比使用+操作符的字符串连接稍微慢一点。
文件:StringTest.java
class StringTest {
public static void main(String[] args) {
String formatString = "Hi %s; Hi to you %s";
long start = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
String s = String.format(formatString, i, +i * 2);
}
long end = System.currentTimeMillis();
System.out.println("Format = " + ((end - start)) + " millisecond");
start = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
String s = "Hi " + i + "; Hi to you " + i * 2;
}
end = System.currentTimeMillis();
System.out.println("Concatenation = " + ((end - start)) + " millisecond");
start = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
StringBuilder bldString = new StringBuilder("Hi ");
bldString.append(i).append("Hi to you ").append(i * 2).toString();
}
end = System.currentTimeMillis();
System.out.println("String Builder = " + ((end - start)) + " millisecond");
}
}
Shell命令:(编译并运行StringTest 5次)
> javac StringTest.java
> sh -c "for i in \$(seq 1 5); do echo \"Run \${i}\"; java StringTest; done"
结果:
Run 1
Format = 1290 millisecond
Concatenation = 115 millisecond
String Builder = 130 millisecond
Run 2
Format = 1265 millisecond
Concatenation = 114 millisecond
String Builder = 126 millisecond
Run 3
Format = 1303 millisecond
Concatenation = 114 millisecond
String Builder = 127 millisecond
Run 4
Format = 1297 millisecond
Concatenation = 114 millisecond
String Builder = 127 millisecond
Run 5
Format = 1270 millisecond
Concatenation = 114 millisecond
String Builder = 126 millisecond
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