JDK8 with -source 1.7 [默认方法] [英] JDK8 with -source 1.7 [Default Methods]

问题描述

我有以下类。

  public class ZonedDateTimeToInstant {
 
 public static void main（final String [] args）
 throws NoSuchMethodException {
 
 assert ChronoZonedDateTime.class.isAssignableFrom（ZonedDateTime.class）; 
 final方法toInstant 
 = ChronoZonedDateTime.class.getMethod（toInstant）; 
 
 final ZonedDateTime now = ZonedDateTime.now（）; 
 final Instant instant = now.toInstant（）; 
 
 System.out.println（instant）; 
} 
} 
  

只是编译得很好。

 & javac ZonedDateTimeToInstant.java 
  

并且失败， -source 1.7 。

 & javac -source 1.7 ZonedDateTimeToInstant.java 
 ZonedDateTimeToInstant.java:10：错误：找不到符号
 final Instant instant = now.toInstant（）; 
 ^ 
符号：方法toInstant（）
 location：变量现在的类型ZonedDateTime 
 1错误
 1警告
  

这是正常吗？看起来 javac 不能理解 -source 而不是 1.8 。

根据 javac ， javac 仍支持各种 -source release

我已经知道

a href =http://jcp.org/en/jsr/detail?id=310 =nofollow> JSR 310：日期和时间API 仅在Java 8中可用。它有什么重要与 javac ？

  $ cat Java8.java 
 class Java8 {
 public void print（java.io.PrintStream out）{
 out.printf（hello world\\\
）; 
} 
} 
 $ javac Java8.java 
 $ cat Java7.java 
 public class Java7 {
 public static void main（final String [] args ）{
 new Java8（）。print（System.out）; 
} 
} 
 $ javac -source 1.7 -target 1.7 Java7.java 
 warning：[options] bootstrap类路径未与-source配合使用1.7 
 1 warning 
 $ java Java7 
 hello world 
  

结论

As @ Eng.Fouad注意到。问题是该方法是在接口中定义的默认方法。 javac 似乎正在抓住这一点。

  $ cat Java8i.java 
 public interface Java8i {
 default void print（java.io.PrintStream out）{
 out.printf（hello world\\\
）; 
} 
} 
 $ javac Java8i.java 
 $ cat Java8c.java 
 public class Java8c implements Java8i {
} 
 $ javac Java8c .java 
 $ cat Java7i.java 
 public class Java7i {
 public static void main（final String [] args）{
 new Java8c（）。print（System.out） ; 
} 
} 
 $ javac -source 1.7 -target 1.7 Java7i.java 
 warning：[options] bootstrap类路径未与-source配合使用1.7 
 Java7i .java：3：错误：找不到符号
 new Java8c（）。print（System.out）; 
 ^ 
 symbol：method print（PrintStream）
 location：class Java8c 
 1 error 
 1 warning 
   $ p> 
 
 
   javac 应该更有帮助地告诉我。 $ b 
解决方案
 这是一个新的Time / Date API，在Java 8中引入。这就是为什么它不能用Java 7编译。 > 
 
 
 
  toInstant（）是一种默认方法， -source 1.7 不支持默认方法（Java 8的新功能）。
 
I have the following class.
public class ZonedDateTimeToInstant {

    public static void main(final String[] args)
        throws NoSuchMethodException {

        assert ChronoZonedDateTime.class.isAssignableFrom(ZonedDateTime.class);
        final Method toInstant
            = ChronoZonedDateTime.class.getMethod("toInstant");

        final ZonedDateTime now = ZonedDateTime.now();
        final Instant instant = now.toInstant();

        System.out.println(instant);
    }
}
It just compiles fine.
& javac ZonedDateTimeToInstant.java
And it fails with -source 1.7.
& javac -source 1.7 ZonedDateTimeToInstant.java
ZonedDateTimeToInstant.java:10: error: cannot find symbol
    final Instant instant = now.toInstant();
                               ^
  symbol:   method toInstant()
  location: variable now of type ZonedDateTime
1 error
1 warning
Is this normal? It seems that javac doesn't understand JDK classes with -source other than 1.8.

According to javac, javac still supports various -source release options as previous releases did.

Supplement

I already know the JSR 310: Date and Time API is only available in Java 8. What does it matter with javac?
$ cat Java8.java
public class Java8 {
    public void print(java.io.PrintStream out) {
        out.printf("hello world\n");
    }
}
$ javac Java8.java
$ cat Java7.java
public class Java7 {
    public static void main(final String[] args) {
        new Java8().print(System.out);
    }
}
$ javac -source 1.7 -target 1.7 Java7.java
warning: [options] bootstrap class path not set in conjunction with -source 1.7
1 warning
$ java Java7
hello world
Conclusion

As @Eng.Fouad noted. The problem was that the method is a default method defined in an interface. javac seems to be catching that point.
$ cat Java8i.java
public interface Java8i {
    default void print(java.io.PrintStream out) {
        out.printf("hello world\n");
    }
}
$ javac Java8i.java
$ cat Java8c.java
public class Java8c implements Java8i {
}
$ javac Java8c.java
$ cat Java7i.java
public class Java7i {
    public static void main(final String[] args) {
        new Java8c().print(System.out);
    }
}
$ javac -source 1.7 -target 1.7 Java7i.java
warning: [options] bootstrap class path not set in conjunction with -source 1.7
Java7i.java:3: error: cannot find symbol
    new Java8c().print(System.out);
                ^
  symbol:   method print(PrintStream)
  location: class Java8c
1 error
1 warning
javac should've told me more helpfully.
解决方案
This is a new Time/Date API which is introduced in Java 8. That's why it does not compile with Java 7.

toInstant() is a default method, whereas -source 1.7 doesn't support default methods (Java 8's new feature).

                        
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