来自Perl，我肯定错过了在源代码中创建多行字符串的“here-document”方法:
$string = <<"EOF" # create a three-line string
text
text
text
EOF
在Java中，当我从头开始连接多行字符串时，我必须在每一行上使用繁琐的引号和加号。
有什么更好的选择吗?在属性文件中定义我的字符串?
编辑:有两个答案说StringBuilder.append()比加号更可取。谁能详细解释一下他们为什么这么想?在我看来，这一点也不可取。我正在寻找一种方法来解决多行字符串不是一级语言结构这一事实，这意味着我绝对不想用方法调用取代一级语言结构(字符串连接与加号)。
编辑:为了进一步澄清我的问题，我根本不关心性能。我关心的是可维护性和设计问题。
当前回答
一个不错的选择。
import static some.Util.*;
public class Java {
public static void main(String[] args) {
String sql = $(
"Select * from java",
"join some on ",
"group by"
);
System.out.println(sql);
}
}
public class Util {
public static String $(String ...sql){
return String.join(System.getProperty("line.separator"),sql);
}
}
其他回答
一个小技巧。使用这个，我在动态创建的HTML页面中注入javascript
StringBuilder builder = new StringBuilder();
public String getString()
{
return builder.toString();
}
private DropdownContent _(String a)
{
builder.append(a);
return this;
}
public String funct_showhide()
{
return
_("function slidedown_showHide(boxId)").
_("{").
_("if(!slidedown_direction[boxId])slidedown_direction[boxId] = 1;").
_("if(!slideDownInitHeight[boxId])slideDownInitHeight[boxId] = 0;").
_("if(slideDownInitHeight[boxId]==0)slidedown_direction[boxId]=slidedownSpeed; ").
_("else slidedown_direction[boxId] = slidedownSpeed*-1;").
_("slidedownContentBox = document.getElementById(boxId);").
_("var subDivs = slidedownContentBox.getElementsByTagName('DIV');").
_("for(var no=0;no<subDivs.length;no++){").
_(" if(subDivs[no].className=='dhtmlgoodies_content')slidedownContent = subDivs[no];").
_("}").
_("contentHeight = slidedownContent.offsetHeight;").
_("slidedownContentBox.style.visibility='visible';").
_("slidedownActive = true;").
_("slidedown_showHide_start(slidedownContentBox,slidedownContent);").
_("}").getString();
}
使用Properties.loadFromXML (InputStream)。不需要外部库。
比起混乱的代码(因为可维护性和设计是您所关心的)，最好不要使用长字符串。 从读取xml属性开始:
InputStream fileIS = YourClass.class.getResourceAsStream("MultiLine.xml");
Properties prop = new Properies();
prop.loadFromXML(fileIS);
然后，您可以使用您的多行字符串在一个更可维护的方式…
static final String UNIQUE_MEANINGFUL_KEY = "Super Duper UNIQUE Key";
prop.getProperty(UNIQUE_MEANINGFUL_KEY) // "\n MEGA\n LONG\n..."
MultiLine.xml`位于同一个文件夹YourClass:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd">
<properties>
<entry key="Super Duper UNIQUE Key">
MEGA
LONG
MULTILINE
</entry>
</properties>
注:你可以使用<![CDATA[”…"]]>为类似xml的字符串。
一个不错的选择。
import static some.Util.*;
public class Java {
public static void main(String[] args) {
String sql = $(
"Select * from java",
"join some on ",
"group by"
);
System.out.println(sql);
}
}
public class Util {
public static String $(String ...sql){
return String.join(System.getProperty("line.separator"),sql);
}
}
当使用一长串的+时，只会创建一个StringBuilder，除非在编译时确定String，在这种情况下不使用StringBuilder !
StringBuilder唯一更高效的情况是使用多个语句构造String。
String a = "a\n";
String b = "b\n";
String c = "c\n";
String d = "d\n";
String abcd = a + b + c + d;
System.out.println(abcd);
String abcd2 = "a\n" +
"b\n" +
"c\n" +
"d\n";
System.out.println(abcd2);
注意:只创建了一个StringBuilder。
Code:
0: ldc #2; //String a\n
2: astore_1
3: ldc #3; //String b\n
5: astore_2
6: ldc #4; //String c\n
8: astore_3
9: ldc #5; //String d\n
11: astore 4
13: new #6; //class java/lang/StringBuilder
16: dup
17: invokespecial #7; //Method java/lang/StringBuilder."<init>":()V
20: aload_1
21: invokevirtual #8; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
24: aload_2
25: invokevirtual #8; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
28: aload_3
29: invokevirtual #8; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
32: aload 4
34: invokevirtual #8; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
37: invokevirtual #9; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
40: astore 5
42: getstatic #10; //Field java/lang/System.out:Ljava/io/PrintStream;
45: aload 5
47: invokevirtual #11; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
50: ldc #12; //String a\nb\nc\nd\n
52: astore 6
54: getstatic #10; //Field java/lang/System.out:Ljava/io/PrintStream;
57: aload 6
59: invokevirtual #11; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
62: return
为了进一步澄清我的问题，我根本不关心性能。我关心的是可维护性和设计问题。
尽你所能让它清晰简单。
注意:这个答案适用于Java 14及以上版本。
Java 15引入了文本块(多行文字)。详情请看这个答案。
听起来像是要做一个多行文字，这在Java中是不存在的。
你最好的选择是用+ d组合的字符串。人们提到的一些其他选项(StringBuilder, String。format, String.join)只有当你从一个字符串数组开始时才更可取。
考虑一下:
String s = "It was the best of times, it was the worst of times,\n"
+ "it was the age of wisdom, it was the age of foolishness,\n"
+ "it was the epoch of belief, it was the epoch of incredulity,\n"
+ "it was the season of Light, it was the season of Darkness,\n"
+ "it was the spring of hope, it was the winter of despair,\n"
+ "we had everything before us, we had nothing before us";
与StringBuilder:
String s = new StringBuilder()
.append("It was the best of times, it was the worst of times,\n")
.append("it was the age of wisdom, it was the age of foolishness,\n")
.append("it was the epoch of belief, it was the epoch of incredulity,\n")
.append("it was the season of Light, it was the season of Darkness,\n")
.append("it was the spring of hope, it was the winter of despair,\n")
.append("we had everything before us, we had nothing before us")
.toString();
与String.format ():
String s = String.format("%s\n%s\n%s\n%s\n%s\n%s"
, "It was the best of times, it was the worst of times,"
, "it was the age of wisdom, it was the age of foolishness,"
, "it was the epoch of belief, it was the epoch of incredulity,"
, "it was the season of Light, it was the season of Darkness,"
, "it was the spring of hope, it was the winter of despair,"
, "we had everything before us, we had nothing before us"
);
相对于Java8 String.join():
String s = String.join("\n"
, "It was the best of times, it was the worst of times,"
, "it was the age of wisdom, it was the age of foolishness,"
, "it was the epoch of belief, it was the epoch of incredulity,"
, "it was the season of Light, it was the season of Darkness,"
, "it was the spring of hope, it was the winter of despair,"
, "we had everything before us, we had nothing before us"
);
如果您想在特定的系统中使用换行符，您要么需要使用system . lineseparator()，要么可以在String.format中使用%n。
另一种选择是将资源放在文本文件中，只读取该文件的内容。对于非常大的字符串，这将是可取的，以避免不必要地膨胀您的类文件。
