Add generics to the EL API

These are the obvious ones. More generics may be added after further
discussion. See #157

api/src/main/java/jakarta/el/CompositeELResolver.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2019 Oracle and/or its affiliates and others.
+ * Copyright (c) 1997, 2021 Oracle and/or its affiliates and others.
  * All rights reserved.
  * Copyright 2004 The Apache Software Foundation
  *
@@ -468,10 +468,10 @@ public Class<?> getCommonPropertyType(ELContext context, Object base) {
      * @since Jakarta Expression Language 3.0
      */
     @Override
-    public Object convertToType(ELContext context, Object obj, Class<?> targetType) {
+    public <T> T convertToType(ELContext context, Object obj, Class<T> targetType) {
         context.setPropertyResolved(false);
 
-        Object value = null;
+        T value = null;
         for (int i = 0; i < size; i++) {
             value = elResolvers[i].convertToType(context, obj, targetType);
             if (context.isPropertyResolved()) {

api/src/main/java/jakarta/el/ELContext.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2020 Oracle and/or its affiliates and others.
+ * Copyright (c) 1997, 2021 Oracle and/or its affiliates and others.
  * All rights reserved.
  * Copyright 2004 The Apache Software Foundation
  *
@@ -418,13 +418,13 @@ public void exitLambdaScope() {
      *
      * @since Jakarta Expression Language 3.0
      */
-    public Object convertToType(Object obj, Class<?> targetType) {
+    public <T> T convertToType(Object obj, Class<T> targetType) {
         boolean propertyResolvedSave = isPropertyResolved();
         try {
             setPropertyResolved(false);
             ELResolver elResolver = getELResolver();
             if (elResolver != null) {
-                Object res = elResolver.convertToType(this, obj, targetType);
+                T res = elResolver.convertToType(this, obj, targetType);
                 if (isPropertyResolved()) {
                     return res;
                 }

api/src/main/java/jakarta/el/ELProcessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012, 2019 Oracle and/or its affiliates and others.
+ * Copyright (c) 2012, 2021 Oracle and/or its affiliates and others.
  * All rights reserved.
  *
  * This program and the accompanying materials are made available under the
@@ -103,7 +103,7 @@ public Object eval(String expression) {
      * @param expectedType Specifies the type that the resultant evaluation will be coerced to.
      * @return The result of the expression evaluation.
      */
-    public Object getValue(String expression, Class<?> expectedType) {
+    public <T> T getValue(String expression, Class<T> expectedType) {
         ValueExpression exp = factory.createValueExpression(elManager.getELContext(), bracket(expression), expectedType);
         return exp.getValue(elManager.getELContext());
     }

api/src/main/java/jakarta/el/ELResolver.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2019 Oracle and/or its affiliates and others.
+ * Copyright (c) 1997, 2021 Oracle and/or its affiliates and others.
  * All rights reserved.
  * Copyright 2004 The Apache Software Foundation
  *
@@ -326,7 +326,7 @@ public Object invoke(ELContext context, Object base, Object method, Class<?>[] p
      * @return object converted to <code>targetType</code>
      * @throws ELException thrown if errors occur.
      */
-    public Object convertToType(ELContext context, Object obj, Class<?> targetType) {
+    public <T> T convertToType(ELContext context, Object obj, Class<T> targetType) {
         return null;
     }
 }

api/src/main/java/jakarta/el/ExpressionFactory.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2020 Oracle and/or its affiliates and others.
+ * Copyright (c) 1997, 2021 Oracle and/or its affiliates and others.
  * All rights reserved.
  * Copyright 2004 The Apache Software Foundation
  *
@@ -235,7 +235,7 @@ public static ExpressionFactory newInstance(Properties properties) {
      * 
      * @throws ELException thrown if an error results from applying the conversion rules.
      */
-    public abstract Object coerceToType(Object obj, Class<?> targetType);
+    public abstract <T> T coerceToType(Object obj, Class<T> targetType);
 
     /**
      * Retrieves an ELResolver that implements the operations in collections.

api/src/main/java/jakarta/el/TypeConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012, 2019 Oracle and/or its affiliates and others.
+ * Copyright (c) 2012, 2021 Oracle and/or its affiliates and others.
  * All rights reserved.
  *
  * This program and the accompanying materials are made available under the
@@ -87,5 +87,5 @@ public Class<?> getCommonPropertyType(ELContext context, Object base) {
      * @throws ELException thrown if errors occur.
      */
     @Override
-    abstract public Object convertToType(ELContext context, Object obj, Class<?> targetType);
+    abstract public <T> T convertToType(ELContext context, Object obj, Class<T> targetType);
 }

api/src/main/java/jakarta/el/ValueExpression.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2019 Oracle and/or its affiliates and others.
+ * Copyright (c) 1997, 2021 Oracle and/or its affiliates and others.
  * All rights reserved.
  * Copyright 2004 The Apache Software Foundation
  *
@@ -80,7 +80,7 @@ public abstract class ValueExpression extends Expression {
      * @throws ELException if an exception was thrown while performing property or variable resolution. The thrown exception
      * must be included as the cause property of this exception, if available.
      */
-    public abstract Object getValue(ELContext context);
+    public abstract <T> T getValue(ELContext context);
 
     /**
      * Evaluates the expression relative to the provided context, and sets the result to the provided value.