Add VariableProvider API, for dynamically providing variable values

src/main/java/net/objecthunter/exp4j/Expression.java

@@ -30,6 +30,8 @@ public class Expression {
 
     private final Map<String, Double> variables;
 
+    private VariableProvider variableProvider;
+
     private final Set<String> userFunctionNames;
 
     private static Map<String, Double> createDefaultVariables() {
@@ -71,6 +73,10 @@ public Expression setVariable(final String name, final double value) {
         return this;
     }
 
+    public void setVariableProvider(final VariableProvider variableProvider) {
+        this.variableProvider = variableProvider;
+    }
+
     private void checkVariableName(String name) {
         if (this.userFunctionNames.contains(name) || Functions.getBuiltinFunction(name) != null) {
             throw new IllegalArgumentException("The variable name '" + name + "' is invalid. Since there exists a function with the same name");
@@ -172,7 +178,10 @@ public double evaluate() {
                 output.push(((NumberToken) t).getValue());
             } else if (t.getType() == Token.TOKEN_VARIABLE) {
                 final String name = ((VariableToken) t).getName();
-                final Double value = this.variables.get(name);
+                Double value = this.variables.get(name);
+                if (value == null && variableProvider != null) {
+                    value = variableProvider.getVariable(name);
+                }
                 if (value == null) {
                     throw new IllegalArgumentException("No value has been set for the setVariable '" + name + "'.");
                 }

src/main/java/net/objecthunter/exp4j/VariableProvider.java

@@ -0,0 +1,5 @@
+package net.objecthunter.exp4j;
+
+public interface VariableProvider {
+    Double getVariable(String variable);
+}