Difference between hyperledger composer and hyperledger fabric?

Updated answer (as promised in previous post): Hyperledger Composer v0.19.0 (time of writing - Apr 1st 2018) is the default release for building your blockchain business networks using Composer. So when you pull HL/Composer eg via npm install etc) - this uses the current GA edition of Hyperledger Fabric v1.1, the underlying blockchain network infrastructure. This 0.19.0 release includes a fundamental change in the way that Composer business networks are deployed (vis-a-vis previous releases eg 0.16.x - 0.18.x). Business networks are now deployed within Hyperledger Fabric 'as chaincode' - meaning that the business network (rather than Composer runtime) can be agreed by all parties (to the blockchain network) and signed, using a similar management model to non-Composer chaincode. In a later edition, the plan is to add JS requires so that these can be exploited in NodeJS and include other JS functions you've developed - of course, you can already call native Fabric APIs/chaincode functions already, from Composer transaction processors - introduced prior to 0.19.x release). See more info here in the release notes -> https://github.com/hyperledger/composer/releases/tag/v0.19.0

Hyperledger Composer simplifies application development on top of the Hyperledger Fabric blockchain infrastructure.

If you are interested in the blockchain infrastructure, start with the Fabric tutorials.

If you are interested in blockchain applications, start with the Composer tutorials.

The Fabric tutorials also include samples of low level chaincode development (in golang). Composer is a higher level application development framework.

I'd suggest trying both to get an overall view of the capabilities.

As a Java developer, you will also want to check out the Fabric Java SDK for building Java client applications that interact with the blockchain. Java chaincode is also available as of Fabric v1.3.