Java Format体系

Java

信息的格式转换

基类java.text.Format

  • 日期(DateFormat)
  • 数字(NunmberFormat)
  • 消息(MessageFormat)

※ Format非线程安全，每个线程最好独立创建，可以搭配TheadLocal

Format最直观的用法是对象和字符串的相互转换
当然也可以提供位置信息等进行细粒度操作，由子类负责具体实现
内部采用StringBuffer实现

Format.java
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public abstract class Format implements Serializable, Cloneable {
    public final String format (Object obj) {
        return format(obj, new StringBuffer(), new FieldPosition(0)).toString();
    }

    public abstract StringBuffer format(Object obj, StringBuffer toAppendTo, FieldPosition pos);

    public Object parseObject(String source) throws ParseException {
        ParsePosition pos = new ParsePosition(0);
        Object result = parseObject(source, pos);
        if (pos.index == 0) {
            throw new ParseException("Format.parseObject(String) failed",
                pos.errorIndex);
        }
        return result;
    }

    public abstract Object parseObject (String source, ParsePosition pos);
    //...
}

DateFormat

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Format
    |---DateFormat
        |---SimpleDateFormat

DateFormat使用工厂方法获取实例，并且可配置style和locale

  • getTimeInstance
  • getDateInstance
  • getDateTimeInstance
DateFormat.java
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public static final int FULL = 0;
public static final int LONG = 1;
public static final int MEDIUM = 2;
public static final int SHORT = 3;

public static final int DEFAULT = MEDIUM;

//getInstance默认获取datatime实例,并且采取短信息形式
public final static DateFormat getInstance() {
    return getDateTimeInstance(SHORT, SHORT);
}

public final static DateFormat getDateTimeInstance() {
    return get(DEFAULT, DEFAULT, 3, Locale.getDefault());
}

public final static DateFormat getDateTimeInstance(int dateStyle, int timeStyle) {
    return get(timeStyle, dateStyle, 3, Locale.getDefault());
}

默认格式

Sample
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Date d = new Date();
System.out.println(DateFormat.getInstance().format(d));  //17-11-21 下午12:38
System.out.println(DateFormat.getDateTimeInstance().format(d));  //2017-11-21 12:38:16
System.out.println(DateFormat.getDateInstance().format(d));  //2017-11-21
System.out.println(DateFormat.getTimeInstance().format(d));  //12:38:16

NumberFormat

ChoiceFormat

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Format
    |---NumberFormat
            |---ChoiceFormat

根据数字范围来决定相应的字符串，注意超出最低边界会沿用范围结果

直接表达式构造

Sample
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ChoiceFormat cf = new ChoiceFormat("-1#negative|0#zero|0<positive");
System.out.println(cf.format(1)); //positive
System.out.println(cf.format(0)); //zero
System.out.println(cf.format(-0.5)); //negative
System.out.println(cf.format(-1.5)); //negative

也可以拆分构造

Sample
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double[] limits = { -1, 0, 1 };
String[] formats = { "negative", "zero", "positive" };
ChoiceFormat cf = new ChoiceFormat(limits, formats);
System.out.println(cf.format(1)); //positive
System.out.println(cf.format(0)); //zero
System.out.println(cf.format(-0.5)); //negative
System.out.println(cf.format(-1.5)); //negative

MessageFormat

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Format
    |--- MessageFormat

实现简单的占位替换

Sample
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MessageFormat.format("first:{0} second:{1}", "foo", "bar");
//first:foo second:bar

除了指定位置索引，还可以指定一些类型

Sample
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MessageFormat.format("date:{0,date} time:{0,time} progress:{1,number,percent}", new Date(), 0.8);
//date:2017-11-5 time:16:50:28 progress:80%

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