我试图在Ubuntu上安装openjdk-7,但是遇到以下错误:
$ sudo apt-get install openjdk-7-jre
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package openjdk-7-jre is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package openjdk-7-jre has no installation candidate
我想我可能缺少一些仓库或类似的东西,但我找不到任何参考资料来确定位置和内容。