Here are some write-ups and resources for you to understand the fundamentals of JAVA.
The typical hello world program and some variations of it can be learnt by you here.
Every language has some core programming elements. Learn them here.
The resources for concepts related to Data types, variables and constant values are available in form of some explanatory coding problems.
Operators help connect different entities to build expressions and formulas
Simple simple mathematical problems of simple interest, area and circumference of circle, area of triangle etc. are solved here to explain you how java programming can be effectively used for problem solving.
BlueJ IDE, which is popular in schools has a unique technique for getting used input in arguments of main. See some problems related to this technique.
Java programming allows easy methods for changing the instructions flow based on given conditions using if, if-else and switch constructs. See code examples here.
Java programming is powerful in terms of providing repetition of a set of instructions in a controlled manner using things like for, while and do-while loops. Here are sample code.
With conditions and loops one can quickly jump using break and continue instruction. Some good examples are shown here.
Compile Time, Run time and Logical and other different types of errors in a program.