Is there an eval() function in Java?

You can use the ScriptEngine class and evaluate it as a Javascript string.

ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("js");
Object result = engine.eval("4*5");

There may be a better way, but this one works.

There is no standard Java class or method that will do what you want. Your options include:

• Select and use some third-party expression evaluation library. For example JEL or any of the half dozen libraries listed here.

• Wrap the expression in the Java source code for a class with an eval method, send that to the Java compiler, and then load the resulting compiled class.

• Use some scripting language that can be called from Java as an expression evaluator. Possibilities include Javascript¹, BeanShell, and so on. A JSR 223 compliant scripting language implementation can be called via the Scripting API.

• Write your own expression evaluator from scratch.

The first approach is probably simplest. The second and third approaches are a potential security risk if you get the expression to be evaluated from an untrusted user. (Think code injection.)

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^{1 - Javascript in Java SE is a moving target. From Java 6, a version of Mozilla's Rhino Javascript implementation was bundled with Java SE. The in Java 8, it was superseded by Nashorn. In Java 11, Nashorn was deprecated, and finally dropped from the core codebase. As of 2021, both Rhino and Nashorn are being maintained as separate (non-Oracle) products, and Oracle's GraalVM has its own Javascript implementation.}