我已經安裝了 java，但無法使用任何 java 命令！下面顯示了當我嘗試執行 java 命令時得到的結果：
sultan@ubuntu:~$ java
The program 'java' can be found in the following packages:
* default-jre
* gcj-4.6-jre-headless
* openjdk-6-jre-headless
* gcj-4.5-jre-headless
* openjdk-7-jre-headless
Try: sudo apt-get install <selected package>
sultan@ubuntu:~$ sudo apt-get install openjdk-7-jre
[sudo] password for sultan:
Reading package lists... Done
Building dependency tree
Reading state information... Done
openjdk-7-jre is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.