eclipse导入其他包出现错误

eclipse导入其他包出现错误：

package这一行显示错误

 Multiple markers at this line
    - The type java.lang.Object cannot be resolved. It is indirectly referenced from
     required .class files
    - The type java.lang.String cannot be resolved. It is indirectly referenced from
     required .class files

以及io包和util包的错误

The import java.io cannot be resolved

The import java.util cannot be resolved

解决方法：

在package explorer中右键自己的project名，build path，config build path ； jre system library 上面会有一个小叉叉，显示这一项有错误。双击将alternate jre 选择为jdk就ok了!