If I compiled a Java file with the newest JDK, would an older JVM be able to run the .class files?

If I compiled a java file in the newest JDK, would an older JVM be able to run the .class files?

That depends on three things:

• The actual Java versions you are talking about. For instance, a 1.4.0 JVM can run code compiled by a 1.4.2 compiler, but a 1.3.x JVM cannot¹.

• The compilation flags used. There is a -target compiler flag that tells it to generate code that will run on an older (target) JVM. And the -source compiler flag tells it to only accept the older JVM's language features. (This approach won't always work, depending on the Java language features used by your code. But if the code compiles it should work.)

• The library classes that the class file uses. If it uses library classes that don't exist in the older class libraries, then it won't run ... unless you can include a JAR that back-ports the classes². You can avoid this problem by using the -bootclasspath option to compile your code against the APIs of the older version of Java.

Does the bytecode depend on the version of the java it was created with?

Yes, modulo the points above.

^{1 - The Java 8 JVMS states this: "Oracle's Java Virtual Machine implementation in JDK release 1.0.2 supports class file format versions 45.0 through 45.3 inclusive. JDK releases 1.1.* support class file format versions in the range 45.0 through 45.65535 inclusive. For k ≥ 2, JDK release 1.k supports class file format versions in the range 45.0 through 44+k.0 inclusive."}

^{2 - A backport could be problematic too. For example: 1) Things which depend on native code support would most likely require you to implement that native code support. 2) You would most likely need to put any back-port JAR file onto the bootclasspath when you run the code on the older JVM.}

Does the bytecode depend on the version of the java it was created with?

Normally yes. But by using the -source, -target and -bootclasspath options, a 1.7+ compiler can be used to create binaries that are compatible with Java 1.1

First and foremost all java files have a version byte in the class header. Older jvms won't load classes with newer versions, regardless of what features they have.

JVM bytecode is forward compatible between major JVM version, but not backward compatible. However, for the best information you will have to read the JVM release notes because they typically indicate how backward compatible the bytecode is.

Edit clarification since this caused discussion in the comments

JVM bytecode is forward compatible, such that bytecode from one JVM is compatible with with later JVM releases. For example, you can take bytecode from the 1.4 JVM and run it in Java 5 or Java 6 JVM (aside from any sort of regression issues as pointed out by Andrew).

JVM bytecode is not backward compatible between JVMs, such that bytecode from a JVM is not guaranteed to work in a previous release of the JVM, as would be the case if you were attempting to run code compiled for Java 6 in a 1.4.2 JVM.