MessageFormat.format(Object obj)方法主要用途为拼接message信息
用法:
Object[] testArgs = {new String("张三"),new String("大*")}; MessageFormat form = new MessageFormat("{0}是个{1}"); String format = form.format(testArgs); System.out.println(format);
输出结果:
张三是个大*
疑问一:format(testArgs);这里传的参数为Object[]类型
源码为:
/**
* Formats an object to produce a string. This is equivalent to
* <blockquote>
* {@link #format(Object, StringBuffer, FieldPosition) format}<code>(obj,
* new StringBuffer(), new FieldPosition(0)).toString();</code>
* </blockquote>
*
* @param obj The object to format
* @return Formatted string.
* @exception IllegalArgumentException if the Format cannot format the given
* object
*/
public final String format (Object obj) {
return format(obj, new StringBuffer(), new FieldPosition(0)).toString();
}
这里要求参数为Object类型,尝试传Object类型,代码如下:
public static void main(String[] args) {
//Object[] testArgs = {new String("张三"),new String("大*")};
Object testArgs1 = new String("张三"); MessageFormat form = new MessageFormat("{0}是个大*"); String format = form.format(testArgs1); System.out.println(format); }
报错<java.lang.ClassCastException>信息如下
Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to [Ljava.lang.Object;
at java.text.MessageFormat.format(MessageFormat.java:865)
at java.text.Format.format(Format.java:157)
at lijian.test.Test000.main(Test000.java:14)
深入源码分析:
Format中的format (Object obj)实际return的为
format(obj, new StringBuffer(), new FieldPosition(0)).toString();
找到这个方法
/**
* Formats an object and appends the resulting text to a given string
* buffer.
* If the <code>pos</code> argument identifies a field used by the format,
* then its indices are set to the beginning and end of the first such
* field encountered.
*
* @param obj The object to format
* @param toAppendTo where the text is to be appended
* @param pos A <code>FieldPosition</code> identifying a field
* in the formatted text
* @return the string buffer passed in as <code>toAppendTo</code>,
* with formatted text appended
* @exception NullPointerException if <code>toAppendTo</code> or
* <code>pos</code> is null
* @exception IllegalArgumentException if the Format cannot format the given
* object
*/
public abstract StringBuffer format(Object obj,
StringBuffer toAppendTo,
FieldPosition pos);
是一个抽象方法,由于是MessageFormat调用的,所以在MessageFormat类中找其实现方法
// Overrides
/**
* Formats an array of objects and appends the <code>MessageFormat</code>'s
* pattern, with format elements replaced by the formatted objects, to the
* provided <code>StringBuffer</code>.
* This is equivalent to
* <blockquote>
* <code>{@link #format(java.lang.Object[], java.lang.StringBuffer, java.text.FieldPosition) format}((Object[]) arguments, result, pos)</code>
* </blockquote>
*
* @param arguments an array of objects to be formatted and substituted.
* @param result where text is appended.
* @param pos On input: an alignment field, if desired.
* On output: the offsets of the alignment field.
* @exception IllegalArgumentException if an argument in the
* <code>arguments</code> array is not of the type
* expected by the format element(s) that use it.
*/
public final StringBuffer format(Object arguments, StringBuffer result,
FieldPosition pos)
{
return subformat((Object[]) arguments, result, pos, null);
}
这里对arguments参数做了一个强转...强转为了Object[]方法,怪不得报错了
由于业务需要,这里必须传Object,而不能传数组,这如何解决?
掩耳盗铃方案:
public static void main(String[] args) {
Object[] testArgs = {new String("张三"),new String("大*")};
Object testArgs1 = testArgs; MessageFormat form = new MessageFormat("{0}是个{1}"); String format = form.format(testArgs1); System.out.println(format); }
将Object[]数组转为Object类型,谁让它是根类呢,啥都能装
到MessageFormat类中的format方法时强转回Object[]数组,就不会报错了
运行结果:
你是个大*