Why java uses both compiler and interpreter?

The JVM runtime executes bytecode by interpreting it or using a just-in-time compiler (JIT). The Java compiler translates Java source code into a platform independent language called Java bytecode. Although bytecode is similar to machine language, but it is not the machine language of any actual computer. A Java interpreter or a just-in-time compiler (JIT) is used to run the compiled Java bytecode . Each platform needs its own Java bytecode interpreter, but all these interpreters interpret the same bytecode . JIT compiling, not interpreting, is used in most JVMs today to achieve greater speed.

The JIT compiler reads the bytecodes in many sections and compiles them dynamically into machine language so the program can run faster. A traditional interpreted virtual machine will simply interpret the bytecode, generally with much lower performance. Currently JIT is used by most implementations of the HotSpot Java Virtual Machine.