JDK

On Mac OS, /usr/bin/java and friends are stubs that delegate to the real JDK commands. These stubs respect the setting of your JAVA_HOME environment variable, but for this to work you need to install the JDK (from http://www.oracle.com/technetwork/java/javase/downloads/index.html) as opposed to the JRE (from http://java.com).

The JDK installs into /Library/Java/JavaVirtualMachines/jdk1.7.0_NN.jdk (for whatever value of NN), so set your JAVA_HOME environment variable to /Library/Java/JavaVirtualMachines/jdk1.7.0_NN.jdk/Contents/Home to make /usr/bin/java use 1.7. You can switch back to 1.6 simply by pointing your JAVA_HOME to /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home instead. You can use the /usr/libexec/java_home tool to find the right value automatically, for example to make /usr/bin/java use Java 7 you can do

export JAVA_HOME=`/usr/libexec/java_home -v '1.7*'`

and to make it use Java 6 you can do

export JAVA_HOME=`/usr/libexec/java_home -v '1.6*'`

The same applies to Java 8 (using -v '1.8*'). This will pick up the latest installed JDK for the relevant major version, you don’t need to remember to change the NN by hand when you install an update.

JRE

If you want to run the 1.7 or 1.8 JRE from the command line, it can be found in /Library/Internet Plug-Ins/JavaAppletPlugin.plugin/Contents/Home/bin/java. This is a fixed path and you can only have one “public” JRE installed at any given time.

$ /Library/Internet\ Plug-Ins/JavaAppletPlugin.plugin/Contents/Home/bin/java -version
java version "1.7.0_13"
Java(TM) SE Runtime Environment (build 1.7.0_13-b20)
Java HotSpot(TM) 64-Bit Server VM (build 23.7-b01, mixed mode)

You could use a shell alias in your .bashrc

alias java_jre="/Library/Internet\ Plug-Ins/JavaAppletPlugin.plugin/Contents/Home/bin/java"