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Building dependency tree      

Reading state information... Done

Package openjdk-7-jdk is not available, butis 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-jdk' has noinstallation candidate

1.    安装openjdk-7-jdk

是因为Ubuntu16.04的安装源已经默认没有openjdk7了，所以要自己手动添加仓库，如下：

(1)   sudoadd-apt-repository ppa:openjdk-r/ppa

添加oracle openjdk ppa source，提示：

sudo: add-apt-repository: command not found

改为用add-apt-repository ppa:openjdk-r/ppa，提示

The program 'add-apt-repository' iscurrently not installed. You can install it by typing:

apt install software-properties-common

很清楚地告诉我们没有安装'add-apt-repository，接着采用apt install software-properties-common安装。

(2)   sudoapt-get update更新软件列表

这个命令，会访问源列表(/etc/apt/sources.list)里的每个网址，并读取软件列表，然后保存在本地电脑

sudo apt-get install openjdk-7-jdk  // OpenJdk 7安装

2.    查看安装openjdk版本

已安装了openjdk-7-jdk和openjdk-8-jdk

java -version查看当前选中的版本，内容如下：

openjdk version "1.8.0_151"

OpenJDK Runtime Environment (build1.8.0_151-8u151-b12-0ubuntu0.16.04.2-b12)

OpenJDK 64-Bit Server VM (build 25.151-b12,mixed mode)

通过ll /etc/alternatives/java命令看结果：

lrwxrwxrwx 1 root root 46 Feb 17 22:05/etc/alternatives/java -> /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java*

查看已注册列表update-alternatives --display java

java - manual mode

 link best version is /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java

 link currently points to /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java

  linkjava is /usr/bin/java

 slave java.1.gz is /usr/share/man/man1/java.1.gz

/usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java- priority 1071

 slave java.1.gz:/usr/lib/jvm/java-7-openjdk-amd64/jre/man/man1/java.1.gz

/usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java- priority 1081

 slave java.1.gz:/usr/lib/jvm/java-8-openjdk-amd64/jre/man/man1/java.1.gz

3.    选择jdk版本

sudo update-alternatives --config java

sudo update-alternatives --config javac

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