threading_model.md: Add Java code example to Objects Validity

topics/basics/architectural_overview/threading/threading_model.md

@@ -268,6 +268,10 @@ In all other cases, it is required to wrap read operation in a read action with
 The read objects aren't guaranteed to survive between several consecutive read actions.
 Whenever starting a read action, check if the PSI/VFS/project/module is still valid.
 Example:
+
+<tabs group="languages">
+<tab title="Kotlin" group-key="kotlin">
+
 ```kotlin
 val virtualFile = runReadAction { // read action 1
   // read a virtual file
@@ -280,6 +284,25 @@ val psiFile = runReadAction { // read action 2
 }
 ```
 
+</tab>
+<tab title="Java" group-key="java">
+
+```java
+VirtualFile virtualFile = ReadAction.compute(() -> {
+    // read a virtual file
+});
+// do other time-consuming work...
+PsiFile psiFile = ReadAction.compute(() -> { // read action 2
+    if (virtualFile.isValid()) { // check if the virtual file is valid
+        return PsiManager.getInstance(project).findFile(virtualFile);
+    }
+    return null;
+});
+```
+
+</tab>
+</tabs>
+
 Between executing first and second read actions, another thread could invalidate the virtual file:
 
 ```mermaid