Правки.

react_online_calculator/src/App.js

@@ -1,8 +1,16 @@
-import React from 'react';
+import React, { useEffect } from 'react';
 
 function App() {
+
+  useEffect(() =>{
+    fetch('http:/localhost:8080/String')
+    .then(response => response.text())
+    .then(result => console.log(result))
+  }, []);
+
   return (
     <div className="App">
+      
     </div>
   );
 }

spring_online_calculator/src/main/java/com/example/spring_online_calculator/SpringOnlineCalculatorApplication.java

@@ -2,6 +2,7 @@ package com.example.spring_online_calculator;
 
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.web.bind.annotation.CrossOrigin;
 import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.RequestParam;
 
@@ -13,8 +14,9 @@ public class SpringOnlineCalculatorApplication {
 		SpringApplication.run(SpringOnlineCalculatorApplication.class, args);
 	}
 
+	@CrossOrigin
 	@GetMapping("/String")
-	public String PrintName(@RequestParam(value = "str", defaultValue = "Иван") String str) {
-		return String.format("Вы ввели %s!", str);
+	public String PrintName(@RequestParam(value = "Егор", defaultValue = "Иван") String str) {
+		return String.format("Привет, %s!", str);
 	}
 }