Linux 安裝 JDK 1.5 完整流程：下載、解壓、update-alternatives 切換預設版本

update-alternatives --display java
update-alternatives --install /usr/bin/java java /opt/jdk/bin/java 60
update-alternatives --install /usr/bin/java java /usr/lib/jvm/jre-1.6.0-openjdk/bin/java 40
update-alternatives --config java  # 選擇預設

update-alternatives --display javac
update-alternatives --install /usr/bin/javac javac /opt/jdk/bin/javac 60
update-alternatives --config javac