The spec for method javax.script.SimpleBindings.put(String, Object) says:
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public Object put(String name,
                  Object value)
    Sets the specified key/value in the underlying map field.

    Specified by:
        put in interface Map<String,Object>
    Specified by:
        put in interface Bindings

    Parameters:
        name - Name of value
        value - Value to set.
    Returns:
        Previous value for the specified key. Returns null if key was previously unset.
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    Throws:
        NullPointerException - if the name is null.
        IllegalArgumentException - if the name is empty
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In fact, the method does not return previous value for the specified key. Here is a short example:

import java.util.HashMap;
import javax.script.SimpleBindings;

public class Test {
    
    public static void main(String[] argv) {
        HashMap<String, Object> map = new HashMap<String, Object>();
        SimpleBindings binds = new SimpleBindings();
        map.put("key", "oldvalue");
        binds.put("key", "oldvalue");
        System.out.println("HashMap returned: " + map.put("key", "newvalue"));
        System.out.println("SimpleBindings returned: " +
                binds.put("key", "newvalue"));
    }
}

Results:
HashMap returned: oldvalue
SimpleBindings returned: newvalue


As you can see the method SimpleBindings.put(String,Object) returns "newvalue" instead of "oldvalue".