Turning string into float Java

Question

I am new to Java so this may be a simple question.

I have made a calculator where the user clicks buttons and it enters the text into a JTextField.

I have used the .getText() method and saved it into a String.

Using System.out.println() I have seen that the String is "8*7*7-8/4", obviously if I could just execute this and get the answer 390 then this would be far easier than splitting it up and making it more complicated than it seems.

Thanks Matt

You could use a stack to convert infix to postfix to get the correct order of operations. — C.B., Apr 03 '14 at 14:30
String to float: http://stackoverflow.com/questions/7552660/java-convert-float-to-string-and-string-to-float — Omar Hrynkiewicz, Apr 03 '14 at 14:34
One possible way to use javascript like eval. http://stackoverflow.com/a/2605051/1193808 — Subir Kumar Sao, Apr 03 '14 at 14:42

score 1 · Answer 1 · edited May 23 '17 at 10:30

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You can actually evaluate the expression "8*7*7-8/4" directly using the built-in Java script engine. Have a look at this post.

Also have a look at this codereview post that explains some good points on how to evaluate a math expression present in a string.

edited May 23 '17 at 10:30

Community

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answered Apr 03 '14 at 14:34

anirudh

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I am referring to the `javax.script.ScriptEngineManager` and `javax.script.ScriptEngine` classes as shown in the link to the post I have given. Its built-in in JDK 1.6. – anirudh Apr 03 '14 at 14:50

score 0 · Answer 2 · edited May 23 '17 at 12:25

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You could do this to get the value of a float from a string:

Float.valueOf(string);

So you could do this, whenever a user inputs a number:

//method when user inputs a number, the String input will be what the user adds
float number = Float.valueOf(input);

Also, you could do something like this:

ScriptEngineManager mgr = new ScriptEngineManager();
ScriptEngine engine = mgr.getEngineByName("JavaScript");
String input = "40+2";
System.out.println(engine.eval(input));

Above code from: Evaluating a math expression given in string form

edited May 23 '17 at 12:25

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answered Apr 03 '14 at 14:51

Jojodmo

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Hi I tried this and I got Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input string: "3*6" – Matt.Pinder Apr 03 '14 at 14:56
@user3494178 The first code, `Float.valueOf(string);`, only works if your doing a single number, like `5.6`, or `8`. Stuff like `8 + 3` and `5.6 * 22.1` won't work here. Yet, the bottom code, with`ScriptEngineManager mgr = new ScriptEngineManager();` will evaluate expressions like `3 * 6`, or `8 + 5` for you – Jojodmo Apr 04 '14 at 03:42

score 0 · Answer 3 · answered Apr 03 '14 at 15:16

Add jar file jav8-jsr223-win-amd64-0.6 to the classpath for Java8 or corresponding jar as per JDK version and write the below code

String val = "8*7*7-8/4";//make it to get from field.getText()

ScriptEngineManager engineManager = new ScriptEngineManager();
ScriptEngine engine = engineManager.getEngineByName("JavaScript");
Object result = engine.eval(val);

System.out.println(result);

Turning string into float Java

3 Answers3