字符串格式中的命名占位符

在Python中，格式化字符串时，我可以按名称而不是按位置填充占位符，如下所示:

print "There's an incorrect value '%(value)s' in column # %(column)d" % \
  { 'value': x, 'column': y }

我想知道这在Java中是否可能(希望没有外部库)?

当前回答

有Java插件使用字符串插值在Java(像在Kotlin, JavaScript)。支持Java 8,9,10,11…https://github.com/antkorwin/better-strings

在字符串字面量中使用变量:

int a = 3;
int b = 4;
System.out.println("${a} + ${b} = ${a+b}");

使用表达式:

int a = 3;
int b = 4;
System.out.println("pow = ${a * a}");
System.out.println("flag = ${a > b ? true : false}");

使用功能:

@Test
void functionCall() {
    System.out.println("fact(5) = ${factorial(5)}");
}

long factorial(int n) {
    long fact = 1;
    for (int i = 2; i <= n; i++) {
        fact = fact * i;
    }
    return fact;
}

更多信息，请阅读项目README。

2020-06-15 20:12:41

其他回答

I created also a util/helper class (using jdk 8) which can format a string an replaces occurrences of variables. For this purpose I used the Matchers "appendReplacement" method which does all the substitution and loops only over the affected parts of a format string. The helper class isn't currently well javadoc documented. I will changes this in the future ;) Anyway I commented the most important lines (I hope). public class FormatHelper { //Prefix and suffix for the enclosing variable name in the format string. //Replace the default values with any you need. public static final String DEFAULT_PREFIX = "${"; public static final String DEFAULT_SUFFIX = "}"; //Define dynamic function what happens if a key is not found. //Replace the defualt exception with any "unchecked" exception type you need or any other behavior. public static final BiFunction<String, String, String> DEFAULT_NO_KEY_FUNCTION = (fullMatch, variableName) -> { throw new RuntimeException(String.format("Key: %s for variable %s not found.", variableName, fullMatch)); }; private final Pattern variablePattern; private final Map<String, String> values; private final BiFunction<String, String, String> noKeyFunction; private final String prefix; private final String suffix; public FormatHelper(Map<String, String> values) { this(DEFAULT_NO_KEY_FUNCTION, values); } public FormatHelper( BiFunction<String, String, String> noKeyFunction, Map<String, String> values) { this(DEFAULT_PREFIX, DEFAULT_SUFFIX, noKeyFunction, values); } public FormatHelper(String prefix, String suffix, Map<String, String> values) { this(prefix, suffix, DEFAULT_NO_KEY_FUNCTION, values); } public FormatHelper( String prefix, String suffix, BiFunction<String, String, String> noKeyFunction, Map<String, String> values) { this.prefix = prefix; this.suffix = suffix; this.values = values; this.noKeyFunction = noKeyFunction; //Create the Pattern and quote the prefix and suffix so that the regex don't interpret special chars. //The variable name is a "\w+" in an extra capture group. variablePattern = Pattern.compile(Pattern.quote(prefix) + "(\\w+)" + Pattern.quote(suffix)); } public static String format(CharSequence format, Map<String, String> values) { return new FormatHelper(values).format(format); } public static String format( CharSequence format, BiFunction<String, String, String> noKeyFunction, Map<String, String> values) { return new FormatHelper(noKeyFunction, values).format(format); } public static String format( String prefix, String suffix, CharSequence format, Map<String, String> values) { return new FormatHelper(prefix, suffix, values).format(format); } public static String format( String prefix, String suffix, BiFunction<String, String, String> noKeyFunction, CharSequence format, Map<String, String> values) { return new FormatHelper(prefix, suffix, noKeyFunction, values).format(format); } public String format(CharSequence format) { //Create matcher based on the init pattern for variable names. Matcher matcher = variablePattern.matcher(format); //This buffer will hold all parts of the formatted finished string. StringBuffer formatBuffer = new StringBuffer(); //loop while the matcher finds another variable (prefix -> name <- suffix) match while (matcher.find()) { //The root capture group with the full match e.g ${variableName} String fullMatch = matcher.group(); //The capture group for the variable name resulting from "(\w+)" e.g. variableName String variableName = matcher.group(1); //Get the value in our Map so the Key is the used variable name in our "format" string. The associated value will replace the variable. //If key is missing (absent) call the noKeyFunction with parameters "fullMatch" and "variableName" else return the value. String value = values.computeIfAbsent(variableName, key -> noKeyFunction.apply(fullMatch, key)); //Escape the Map value because the "appendReplacement" method interprets the $ and \ as special chars. String escapedValue = Matcher.quoteReplacement(value); //The "appendReplacement" method replaces the current "full" match (e.g. ${variableName}) with the value from the "values" Map. //The replaced part of the "format" string is appended to the StringBuffer "formatBuffer". matcher.appendReplacement(formatBuffer, escapedValue); } //The "appendTail" method appends the last part of the "format" String which has no regex match. //That means if e.g. our "format" string has no matches the whole untouched "format" string is appended to the StringBuffer "formatBuffer". //Further more the method return the buffer. return matcher.appendTail(formatBuffer) .toString(); } public String getPrefix() { return prefix; } public String getSuffix() { return suffix; } public Map<String, String> getValues() { return values; } } You can create a class instance for a specific Map with values (or suffix prefix or noKeyFunction) like: Map<String, String> values = new HashMap<>(); values.put("firstName", "Peter"); values.put("lastName", "Parker"); FormatHelper formatHelper = new FormatHelper(values); formatHelper.format("${firstName} ${lastName} is Spiderman!"); // Result: "Peter Parker is Spiderman!" // Next format: formatHelper.format("Does ${firstName} ${lastName} works as photographer?"); //Result: "Does Peter Parker works as photographer?" Further more you can define what happens if a key in the values Map is missing (works in both ways e.g. wrong variable name in format string or missing key in Map). The default behavior is an thrown unchecked exception (unchecked because I use the default jdk8 Function which cant handle checked exceptions) like: Map<String, String> map = new HashMap<>(); map.put("firstName", "Peter"); map.put("lastName", "Parker"); FormatHelper formatHelper = new FormatHelper(map); formatHelper.format("${missingName} ${lastName} is Spiderman!"); //Result: RuntimeException: Key: missingName for variable ${missingName} not found. You can define a custom behavior in the constructor call like: Map<String, String> values = new HashMap<>(); values.put("firstName", "Peter"); values.put("lastName", "Parker"); FormatHelper formatHelper = new FormatHelper(fullMatch, variableName) -> variableName.equals("missingName") ? "John": "SOMETHING_WRONG", values); formatHelper.format("${missingName} ${lastName} is Spiderman!"); // Result: "John Parker is Spiderman!" or delegate it back to the default no key behavior: ... FormatHelper formatHelper = new FormatHelper((fullMatch, variableName) -> variableName.equals("missingName") ? "John" : FormatHelper.DEFAULT_NO_KEY_FUNCTION.apply(fullMatch, variableName), map); ... For better handling there are also static method representations like: Map<String, String> values = new HashMap<>(); values.put("firstName", "Peter"); values.put("lastName", "Parker"); FormatHelper.format("${firstName} ${lastName} is Spiderman!", map); // Result: "Peter Parker is Spiderman!"

2018-08-23 15:38:51

在编写本文时，Java中还没有内置任何东西。我建议编写自己的实现。我的偏好是一个简单流畅的构建器接口，而不是创建一个映射并将其传递给函数——你最终会得到一个漂亮的连续代码块，例如:

String result = new TemplatedStringBuilder("My name is {{name}} and I from {{town}}")
   .replace("name", "John Doe")
   .replace("town", "Sydney")
   .finish();

下面是一个简单的实现:

class TemplatedStringBuilder {

    private final static String TEMPLATE_START_TOKEN = "{{";
    private final static String TEMPLATE_CLOSE_TOKEN = "}}";

    private final String template;
    private final Map<String, String> parameters = new HashMap<>();

    public TemplatedStringBuilder(String template) {
        if (template == null) throw new NullPointerException();
        this.template = template;
    }

    public TemplatedStringBuilder replace(String key, String value){
        parameters.put(key, value);
        return this;
    }

    public String finish(){

        StringBuilder result = new StringBuilder();

        int startIndex = 0;

        while (startIndex < template.length()){

            int openIndex  = template.indexOf(TEMPLATE_START_TOKEN, startIndex);

            if (openIndex < 0){
                result.append(template.substring(startIndex));
                break;
            }

            int closeIndex = template.indexOf(TEMPLATE_CLOSE_TOKEN, openIndex);

            if(closeIndex < 0){
                result.append(template.substring(startIndex));
                break;
            }

            String key = template.substring(openIndex + TEMPLATE_START_TOKEN.length(), closeIndex);

            if (!parameters.containsKey(key)) throw new RuntimeException("missing value for key: " + key);

            result.append(template.substring(startIndex, openIndex));
            result.append(parameters.get(key));

            startIndex = closeIndex + TEMPLATE_CLOSE_TOKEN.length();
        }

        return result.toString();
    }
}

2019-10-16 19:08:36

这是一个旧的线程，但只是为了记录，你也可以使用Java 8风格，像这样:

public static String replaceParams(Map<String, String> hashMap, String template) {
    return hashMap.entrySet().stream().reduce(template, (s, e) -> s.replace("%(" + e.getKey() + ")", e.getValue()),
            (s, s2) -> s);
}

用法:

public static void main(String[] args) {
    final HashMap<String, String> hashMap = new HashMap<String, String>() {
        {
            put("foo", "foo1");
            put("bar", "bar1");
            put("car", "BMW");
            put("truck", "MAN");
        }
    };
    String res = replaceParams(hashMap, "This is '%(foo)' and '%(foo)', but also '%(bar)' '%(bar)' indeed.");
    System.out.println(res);
    System.out.println(replaceParams(hashMap, "This is '%(car)' and '%(foo)', but also '%(bar)' '%(bar)' indeed."));
    System.out.println(replaceParams(hashMap, "This is '%(car)' and '%(truck)', but also '%(foo)' '%(bar)' + '%(truck)' indeed."));
}

输出将是:

This is 'foo1' and 'foo1', but also 'bar1' 'bar1' indeed.
This is 'BMW' and 'foo1', but also 'bar1' 'bar1' indeed.
This is 'BMW' and 'MAN', but also 'foo1' 'bar1' + 'MAN' indeed.

2016-10-06 17:48:58

不完全是，但你可以使用MessageFormat多次引用一个值:

MessageFormat.format("There's an incorrect value \"{0}\" in column # {1}", x, y);

上面的事情也可以用string .format()来完成，但是如果你需要构建复杂的表达式，我发现messageFormat语法更干净，而且你不需要关心你放入字符串中的对象的类型

2010-02-18 06:30:06